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If Mars does not have active plate tectonics how does it have mountains that vastly overshadow Earth's mountains?

Olympus Mons is a shield volcano thought to be produced by a hot spot which means it is very similar to Hawaii. The rest of the mountains have a similar process (however this can be debated).

If earth's mountains are formed by shifting tectonic plates, are mountains on Mars formed by the same process?

[Here's a great video from a NASA employee](_URL_0_) that outlines the various processes that form mountains on Mars. In short, no, there is no evidence for plate tectonics on mars. The mountains are caused by impact events, and occasionally by volcanism--which can occur without a plate tectonic system, similar to Hawaii or Yellowstone (hotspots). Like /u/Gila_Monster mentioned, Olympus Mons is a huge volcano.

Is a star visible from an infinite number of positions? If so does that mean it's emitting an infinite number of photons? This doesn't seem possible.

A finite number of photons are emitted, and a star is visible from all directions, just not at every point in time. For a small photon detector it is possible to watch a small section of a star's surface and count photons are they enter one at a time, with some time periods having no photons entering as one was not emitted in that direction at a given point in time. Or for stars very far away even even observing the whole star the photon rate is small. This is why hubble uses very long exposure times to find the furthest stars. [link](_URL_0_)

Why do humans not grow facial hair until puberty?

Thickness of body and facial hair in humans depends on levels of androgens like testosterone in the blood system. Humans experience an increase in androgen levels as part of the general change in hormone levels during puberty. As a result, those hairs grow in thicker.

Why does facial hair start growing during/after puberty yet eyebrows and eyelashes etc. are there from the start?

It is a different type of hair called [Androgenic hair](_URL_0_) which doesn't start growing until puberty. It is distinct from vellus hair (the fine hair all over your body) and head hair/eyebrow hair/eyelashes.

Why, if we get a new set of skin every 80 days or so, does skin ever become wrinkled and old?

Collagen and elastase degradation in the skin matrix. These proteins give skin its flexible nature. Over time the protein is no longer secreted as readily by fibroblasts and the existing matrix begin to degrade, resulting in looser skin.

Do blind since born people dream? If so , what do they dream about since they have not seen any real shapes and colors before?

They do dream, but their dreams tend to involve experiences similar to their waking lives, with sounds, smells, and physical sensations but no visual information. By contrast, people who were sighted but lost it during their lives often have full sight in their dreams, since they are familiar with that sensory experience.

Why does bread becoming soggy when heated in a microwave and crispy when toasted in an oven?

I would think its because bread is relatively moist, and microwaves are particularly efficient at heating water. Toasting it heats the water out of it, microwaving heats the water in it.

Why does toasted bread (like pizza or toast) get soft and soggy-like when heated in a microwave?

I've seen this question 6 times this week, check: _URL_0_ _URL_1_ edit: didn't mean that to sound dinkish, I just thought it was strange that this question keeps showing up so regularly. People really love toast.

If the Big Bang originated from a point, how is it that we see the universe grow younger when we look at it from many different points around the globe? Shouldn't we expect to see the universe simply "cut off" at the line marking where the universe still hasn't had time to expand into?

I'm a layman, although I frequent /r/askscience so I've picked up on a few common pitfalls. What you've heard is a simplification. The thing is, the explanation of a super dense point is an easy way of expressing how densely packed matter was in the early stage of the Universe, but the Universe itself was still as infinite as it is today. It was never concentrated into a single point, it was just very very dense. The "explosion" or the big bang then refers to the rapid expansion of space itself, which is still happening today albeit at a far slower pace. This expansion of space happens everywhere. As such, there isn't a center, and there never was.

When we look out at the sky, why do we see so little stars?

Signal to noise ratio. If the light from a city shining off the atmosphere directs 100 brightness units toward your eye from the direction of the star, and the star directs 1 brightness unit towards your eye, you cannot distinguish that the star is there. It is same reason you cannot see anything if someone shines a flashlight into your eye, it overwhelms all the signal that you normally receive.

Why do women have multiple and longer orgasms than men?

Sexually, humans are built much like bonobos. Their females are also always "in heat" and multiorgasmic. Bonobo gals use sex, including group sex, to stop fights and defuse arguments among the males. I have no idea how much of that applied to early humans, though.

What is the maximum mass/size a Star can reach?

The weird thing is that observation and math disagree on this. The math says that anything bigger than about 150 solar masses can't exist because outward fusion pressure would blow off the outer layers before they could coalesce. Observation says that there are stars bigger than that, and margin of error doesn't cover it. So either the math is missing something that allows for bigger stars to exist, or the observations are wrong.

Why does it take several minutes for our eyes to adjust to the dark, when our pupils shrink so quickly?

It's not just the pupil that helps us adjust to bright & dark situations. There is a chemical called Rhodopsin that detects when photons (light) hits it. The light breaks down the Rhodopsin, and it takes a while for it to reform/replenish. So when it's bright and you go into the dark (or stop looking at a bright light when it's dark around you), the time it takes for you to adjust, is just the time that it takes for the Rhodopsin to replenish.

Is there any scientific data that supports the idea that stress causes one's hair to turn grey?

"There is evidence that local expression of stress hormones mediate the signals instructing melanocytes to deliver melanin to keratinocytes," notes Jennifer Lin, a dermatologist who conducts molecular biology research at the Dana-Farber / Harvard Cancer Center in Boston. "Conceivably, if that signal is disrupted, melanin will not deliver pigment to your hair." [link to Scientific American article] (_URL_0_)

Why is it that when you stare and concentrate at a star at night it seems to "blink" and flicker?

_URL_0_ Its because of turbulence in the upper atmosphere of the Earth. Variations in air pressure make the air behave as a lens. It distorts the light which shines through it, and as the pressure changes due to air movement, the distortion changes, causing flickering. Planets don't flicker as much because they are much less of a point source. The path of light from a planet (say Venus) to your eyes is large enough to smooth out the distortion caused by the atmosphere.

Can gill-bearing animals somehow "smell" underwater?

They certainly can! It’s a really important sense for many fishes and it allows them to do things such as find food, avoid predators, locate familiar territory, and identify fish of the same species. Although fish do not use their gills to smell, instead they use olfactory receptors located within pits (nares) on their snout. Smells are carried to these receptors via the flow of water, where it comes into contact with the olfactory rosette (consisting of epithelial folds and lamellae), and upon contact a neural response is triggered in the olfactory lobe of the brain. The shape of the olfactory rosette and the number of lamellae determine how strong the sense of smell is for each type of fish. Seahorses have no lamellae and some eels can have as many as 90. [More info](_URL_0_)

Why do you get an itch?

There are [many causes](_URL_1_) of itching. An itch "out of nowhere" may be related to a cause of which the subject isn't aware. Seemingly random itching usually has some physical cause, unless it is the result of psychological pathology. From [here](_URL_0_): > When the stimuli lands on your skin, it may not bother you at first, but soon it will begin to rub back and forth across your skin. Once the hair or dust scratches your skin's surface layer, receptors in the dermis of the skin will become irritated. In a split second, these receptors send a signal through fibers in the skin to your spinal cord and then up to the cerebral cortex in your brain.

Since atoms are made up of mostly empty space and the universe is as well, how big would the "universe" be if it were a completely condensed mass?

The concept that atoms are largely empty is also not well established, as its unclear if there is such a thing as "empty space". This is compounded by the principles of quantum field theory which suggest quantum fields pervade all of spacetime anyway. We also have good reason to believe that all the matter and radiation in the universe can be squashed up into a single point (being the Big Bang starting point) if you suck out all the "empty space" (taking empty space as that between particles) .

If the universe was confined to a finite space (like a box), would all matter eventually form into a gigantic mass, or would it keep forming stars and smaller planets?

There's this concept called [Schwarzchild radius](_URL_1_). If you cram too much matter into too small a space (smaller than the Schwarzchild radius for that mass) then the gravity would be too strong for even light to escape. We call the resulting object a black hole. Cramming the entire universe into a box would result in a mondo black hole. There is also a lower limit to the mass a star can have and still shine; anything below 75 times the mass of Jupiter has insufficient pressure to fuse hydrogen. I am sorry, but there is no way to realize your dream of having a pet universe in your living room. The best you could get would be a [rough computer simulation](_URL_0_) - which is still fairly impressive.

how does light have momentum but no mass?

The full relativistic equation for energy is E^2 = m^2 c^4 + p^2 c^2. For massive particles at rest (p=0), we get the familiar E = m c^2. For massless particles (like light) we get E = pc. So massless particles have momentum proportional to their energy. Also, wave-particle duality is not a special property of massless things. All particles exhibit this behaviour.

How can light carry momentum if it has no mass?

The notion that only massive objects carry momentum stems from classical, Newtonian, mechanics, which has been shown to just be an approximation for the more complete theory of relativity. In Newtonian physics, momentum is given by the expression p = m * v, which would imply that massless objects have no momentum. But this expression is only a good approximation when velocities are low (with respect to the speed of light). Since light travels at the speed of light, this condition is obviously not met. From relativity, the following expression can be obtained: E^2 = (m c^(2))^2 + (p c)^2 where E is the energy of the particle/object, m is the mass, p the momentum and c the speed of light. For objects that have non-zero mass that are at rest (so zero momentum), this reduces to: E = m c^2 For objects that have zero mass, such as photons, this expression reduces to the expression for momentum of photons: E = p c

How can there be 'terrestrial-sized planets' around this 11.2 byo star?

Elements to form rocky planets, such as silicon, could have been formed in stars that died in a supernova explosion. These stars live a very short amount of time since the more massive a star is during its main-sequence life, the greater its luminosity, the faster it burns through its fuel, and the shorter they live, which can be on the order of a few million years for the most massive stars. So, there could have already been a few generations of star formation to provide material for terrestrial planets by then.

What, from an evolutionary standpoint, does a mosquito have to gain from its bite stinging?

Nothing. Musquito saliva contains proteins that block coagulations and vaso-constriction, and some other molecules, which makes it easier to feed. The immune system mounts a response to these proteins in the area of the sting, which causes inflammation - Which causes the itching. Everything that happens biologically doesn't have to have an evolutionary benefit. The itching is just a by-product.

What happens to the iris when our eyes dilate?

It's the same way anything in our body moves: muscles! Humans have two sets of muscles that control the size of the pupil. The dilator muscles are spoke-like around the rim of the iris, so when they contract, the pupil gets larger. The constrictor muscles run in a circular pattern around the iris and act like a sphincter, so when they contract, the pupil gets smaller. [Image](_URL_0_)

When our pupils dilate, what's the iris doing? Where does the colour "go"?

The iris is a muscle that relaxes, and when it does the pupil is allowed to expand, letting in more light. It constricts to make them smaller. So the “color” goes the same place the muscle in your arm does when you tighten it: right next to the rest of the muscle, just denser / thicker. Edit: I am wrong, people below me are correct.

Why do eggs solidify with the application of heat?

While the [Egg whites](_URL_2_) are mostly water, the liquid contains quite a few different proteins. [Egg Yolks](_URL_1_) also contain quite a bit of protein as well. These proteins undergo a process called [denaturation](_URL_0_) when excessive heat is applied. Proteins are created by chains of amino acids in certain lengths, orders, and arrangements. The heat will ruin these arrangements permanently, creating the solid hard boiled egg (or some middle ground like over easy deliciousness). In any case you can't return the egg to its liquid form after its boiled or fried.

Why do eggs harden with heat? What is the chemistry behind that?

Some people (namely, a biophysicist I know) would say it's more of a physical process, actually. It's pretty cool. As the egg heats up, the proteins inside of it basically go from being aligned to being all tangled up, so it's hard for everything to move around.

Why does moon appear bigger near the horizon? Is it really an illusion? or is there something else to it?

I forget he name of the optical illusion, but it's due to our brain compensating for perceived distance. Things are smaller when they are far away, so if our brain thinks something is far away due to distance clues (such as discoloration, blurriness or objects between for reference) we assume it is large. When the moon is overhead, we have nothing to judge its distance by, so it looks small. However, when it is at the horizon, you can see a vast expanse of ground between you and it (as well as blurriness and discoloration due to the atmosphere), which your brain correctly interprets as the moon being very far away, so you perceive it as being very large (also correct). So it's really not that it looks bigger on the horizon, but smaller when its overhead because we can't judge how far away it is.

Carbon vs. Silicone in transistor construction... which is better?

Just to clarify, *silicone* is different from **silicon**. CPUs are mostly made of silicon, they don't have silicone in them. One key advantage rests on the fact that Si-Si bonds are weaker. It means that the Si lattice can more easily accommodate foreign dopant atoms compared to its carbon analogue. As a result, it is possible to vary the electrical properties of Si with relative ease. Not so with diamond, say.

[Mathematics] If Euclid derived a formula for finding a prime higher than all the other primes, why is us discovering a new prime so important?

You've misunderstood the proof; given the first n primes, it is not, in general, true that multiplying them together and adding 1 will give you a prime number. For example, the first six prime numbers are 2, 3, 5, 7, 11, and 13. If you multiply those together and add 1, you get (2\*3\*5\*7\*11\*13) + 1 = 30031, which isn't prime since 30031 = 59\*509. What Euclid's proof really says is that *either* that number is prime *or* there is a prime, call it p, that divides it. However, p can't be any of the primes you started with, because that would imply that it divides 1 (take the number above and subtract the product of the primes). Thus, there is a prime not on the list. Note also that if you take some arbitrary collection of primes, you can't even guarantee that the "new prime" you get will be bigger than all of the ones with which you started.

Are there many stars in the universe that are not part of any galaxy?

[Here is an article](_URL_0_) of a star being ejected from its galaxy to live out its existence as rogue star. [Here is a release](_URL_1_) about stellar outcasts. EDIT: Updated Link

How do we know that pi is irrational?

If you're into the maths, [wikipedia](_URL_0_) has a nice little article on proofs of pi's irrationality. They're beyond me, but one of the approaches taken is that certain formulas pump out answers that we know to be false if we assume that pi is *rational* (answers such as 'an integer exists that is greater than 0 but less than 1'). This method is called proof by contradiction.

How do companies put such specific expiration dates on products that sometimes go years into the future?

Expiration dates are almost always estimates that are added starting from date of production. A lot of foods can last longer than expiration dates and some things can even last indefinitely past expiration dates, but have them ("best by") to ensure quality. Like for example a lot of baking products like flour can last indefinitely as long as it's kept in an air tight container and is checked before use for things like bugs.

If diseases brought over from Europe managed to wipe out so many Native Americans, why didn't any American diseases wipe out the European explorers?

IIRC Americans had fewer epidemic diseases mostly due to lower population density and fewer domesticated animals. This is one of several discrepancies between the societies covered in Guns, Germs, and Steel if you want to read more. Edit: Moustachoid_T-Rex has pointed out the effects of trade and the total population of the Eurasian continent. As I have heard it explained, the combination of these factors with population density are required for epidemic diseases since they need to be able to spread quickly from person to person and have a large enough population to access that there are always fresh victims (by the time they "return" to an original village they have sufficiently mutated, or the population is sufficiently removed from the selective pressure that the disease can spread again). I think this may be a distinct argument from T-Rex's, so you can read his post for his explanation. To be professional, I should also make clear that this is not my field, and I claim no personal expertise.

How dangerous, if at all, is radiation from microwaves, cellphones, and other electronic devices?

The radiation is non ionizing so there aren't any proven risks. Regulatory agencies still put limits *just in case*, but cellphones and microwaves don't even come near them.

Given a powerful enough telescope, is it possible to pick up 14 billion year old photons and watch the Big Bang unfold?

As /u/Das_Mime pointed out, we can't do it with light, but we can do it (indirectly) with gravitational waves. That's the importance of the recent BICEP2 result. If correct (it probably is), it will let us watch the inflationary epoch (about 10^(-34) sec after the BB) unfold.

Why do flames always burn upwards?

Matter that is heated expands, causing its density to decrease. Due to gravity heated (less dense) matter rises above cooler matter. With flames you see that typical shape because the heated air causes convection currents that pull the flame along. It doesn't always happen this way though - flames with pressure behind them like from a gas soldering torch will keep going in the direction the gas was emitted, and flames in zero-gravity will form balls. Check out Veritasium's video with a lighter on the Vomit Comet - _URL_0_

Why are bubbles always spherical when blown?

There are primarily two forces to consider: pressure and surface tension. There is an outward force due to the difference in pressure between the inside and outside of the bubble, and there is an inward force due to surface tension. The force due to the pressure gradient is trying to increase the volume but the force of surface tension is trying to decrease the surface area. These forces reach equilibrium at the shape which minimizes the surface area for a given volume: the sphere.

Why doesn't the water in alcoholic beverages freeze?

Colligative properties, my friend. The mixture of a solvent (here, water) and a solute (ethanol) will make a solution that has a freezing point below the solvent alone. The more solute there is, the more the freezing point is depressed, so beer will freeze before wine which will freeze before vodka, ignoring other solutes. If you had low-proof booze and got it cold enough, the solution would freeze. But the water would freeze out first, leaving behind a solution with a higher concentration of alcohol. Ta-da! You've fortified the booze without a still! The same phenomena is why you put antifreeze + water mix in your car radiator, put salt on the roads in the winter, and use rock salt (CaCl2) instead of table salt (NaCl) mixed with ice to make ice cream.

Why does vodka freeze only when more water is added to the drink even though water is already a main ingredient?

This is a property of mixtures. If subtance A freezes at some temperature T1, and substance B at another T2, then the freezing temperature of their mixture varies from T1 to T2 as you add more substance B. In essence this property comes from Gibbs free energy considerations.

What would happen if Jupiter suddenly ignited, and became a star?

> I have heard before (can't remember where) that Jupiter is on the threshold of becoming a star [You heard wrong](_URL_0_). The very lowest-mass brown dwarfs, that barely produce any heat at all and can only fuse deuterium, would theoretically be no less than 13 times as massive as Jupiter. And these ultra-low-mass stars would cool to the point that they can not fuse anything on the order of a few million years, many many times younger than our solar system. The lowest mass true stars ([red dwarfs](_URL_1_) which fuse normal hydrogen) are about 75 times the mass of Jupiter.

Why is it so difficult to make a car that doesn't run on gasoline or diesel?

I'm not sure the answer is as scientific as you might think. I'm pretty sure oil companies would rather you depend on them, than you be able to just plug in every time you park your car. Give it a few years, and the infrastructure might be there in enough places to justify the masses purchasing electric only vehicles.

if I just rinse my fruit and vegetables before eating them, does it actually do anything to clean it of pesticides and chemicals?

While pesticides/agrochemicals are generally much more soluble in organic solvents, rinsing with water and rubbing the vegetables will still help quite a bit. This is because most of the pesticides will be absorbed by the dirt/dust on the outside of the vegetable, and if you get this physically removable material off the outside of the plant the majority of the pesticides will go with it. In addition, many pesticides are somewhat water soluble, so rinsing will help directly as well. As to whether or not you are taking in a lot of pesticides, the answer is more up in the air than you think. I do work with sample preparation of fruits and vegetables for pesticide analysis, and it is very hit and miss. 99% of the real world samples tested have no pesticide residue, but the few that do have *a lot* of pesticide. There is a reason I purchase organic fruits and vegetables.

Why are Europe and Asia considered separate continents?

Historical reasons. There isn't a strict definition of what constitutes a continent. edit: It should also be mentioned that in some places, Europe+Asia are considered one continent, and in other places North+South America are considered one continent.

Why do amnesia patients lose their identity, but not motor functions? Such as their ability to walk, talk, ect..

We have many different types of memory. In this case the important distinction is between episodic and procedural memory. The first is the memory of stuff we personally experienced, the later is on how you do stuff. This division is rather handy as it allows you to ride a bike without thinking about all the times you fell down as a kid, learning how to ride a bike. Anyhow these types of memories are independent and can be damaged separately. One interesting case is patients with anterograde amnesia, that is they remember everything up to the accident but they can't form new long term episodic memories, just like in Memento. They can still acquire new skills. There has been a case there as an experiment a patient like this learned to play chess, and developed a certain level of skill in it, but still had no memory of actually playing.

How is Hubble able to see galaxies through all the stars?

Our Milky Way can be imagined like a plate, with us sitting about 2/3 away from the Galactic center. Looking towards the center the density of stars is high (although we do know some galaxies in that direction as well). These stars in the plane form the band of the Milky Way that you can see during dark nights outside of cities. Perpendicular to the galaxy the number of stars along a line of sight is much less and there's no problem at all to look outside. Also, do not forget that the density of stars is very low and that even our Milky Way is very empty - in practice it is dust and gas between stars that is more of a problem for looking far than stars themselves. A good model for the stellar density in our neighborhood is if you shrink the Sun to the size of a typical apple. If our Sun was in Manhattan, the next star, i.e., the next apple, would be in Denver.

Does it make sense to wash dishes with the bubbles because "that's where the soap is?"

This is outside of my panel expertise, but I spent a summer working for a major consumer products company in their soap areas. The primary reason that consumer soap lathers (makes bubbles) is because consumers are conditioned to believe that if the soap doesn't lather, then it isn't cleaning anything. There are many commercial soaps that do not lather, like dishwasher soap, which is why you're always told to never put your sink dish soap in the dishwasher. The bubbles do serve the helpful purpose of letting you know where you haven't properly rinsed yet, and what parts of the dish you have cleaned. And yes, for dish washing soap you're intended to use by hand, bubbles are indicative of a higher local concentration of soap which by their very nature will tend to be at the surface of the water.

Are soap bubbles necessary when cleaning dishes?

No, soap bubbles are not necessary. As long as you have soap in water, it will do its job on dishes. However, bubbles are a helpful indicator that you have enough soap in your water, or that all the detergent molecules are not "busy" encapsulating grease.

Why have mosquitos not evolved methods to suck our blood without leaving an itch?

While I share your disdain, what selection pressure has been applied to their population? They don't itch until after they've flown away, so it's not like we stop the one that bit us from reproducing. And we haven't applied consistent measures against the several species over long enough times to have such a selection pressure.

Why does Venus spin backwards? Is it upside down?

There are a couple theories for this so far. The simplest is basically that something crashed into it, and changed its spin the other way. We can't see the surface well enough to see any craters or such, so we can't confirm nor deny. EDIT: As pointed out below, we can see the surface. My bad. Another theory is that due to the density and mass of Venus's clouds, gravity may have caused a lot of rotational energy to be dissipated through tidal friction. This would have slowed down the planet and then the continued gravitational effects would have taken it to the equilibrium we see today. This theory is based on the fact that the length of 3 Venusian days and 3 Earth years have almost the same length, leading to a possible case of 3:2 tidal resonance. The Moon and the Earth are (I believe, please correct me if I'm wrong) 1:1 tidal resonance, which means 1 Moon day = 1 Earth month.

Why does water start to "roar" just prior to boiling?

The bubbles that form rise into the slightly cooler water above them and quickly condense, collapsing with a 'click'. This happens many times a second with many collapsing water vapour bubbles, producing the roaring noise.

Why does water hiss when it is about to boil, while water already at a rolling boil does not?

This is a result of a phenomenon called cavitation. What happens is that the water that is in direct contact with the heating surface tends to become vapour (steam) and form little steam bubbles. But the moment these tiny bubbles are formed, being less dense than the water around them, they begin to rise in the water column. Very quickly they come into contact with water that is substantially cooler than 100 °C, and as a result the steam changes back into water and the bubble collapses, making a very tiny little bang. When many of tiny bubbles are doing this every second, we hear it as a kind of hiss or roar. Once the water is boiling, the sound stops because the entire water column is at 100 °C and so the bubbles can make it all the way to the surface and actually escape as steam.

How can the distance between two distant galaxies be increasing at a rate greater than the speed of light?

It's the speed *through* space which is limited by 'c', not the speed of light itself. Imagine you're at an airport. The maximum speed that you can move with your heavy suitcase is 10 mph. Now, you get onto the moving walkway... you can walk at 20 mph, with the aid of the belt. Are you exceeding the 10 mph limit? No, because you're only *walking* at 10 mph... it's the ground that's carrying you along, and making your walking speed look faster than it is. This is how it is; moving walkway = expansion of the Universe. The Universe is just the floor that the galaxies move around on, but the floor moves.

How do your fingernails grow laterally along your nailbed while seeming so securely attached?

The nail plate is generated by proliferating cells at the proximal end of the plate called the matrix (sort of under the cuticle). As the new nail is added it is pushed out towards the tip of your finder. It is attached to the nail bed - which is like the epidermis under the nail plate. The nail bed cells also migrate out towards the finger tip along with the nail. At the end of the nail bed is the hyponychium that seals the free edge of the nail plate to prevent stuff from getting shoved under the nail plate. So basically the growing tissue, in multiple layers, is continually being produced and moves out. The only "attachment" is where the progenitor cells sit on a basement membrane over the dermis - all the movement is in differentiated cells in the newly generated tissue. edit: [diagrams and gross pics](_URL_0_)

How are fingernails able to slide along the nailbed as they grow, yet also remain firmly attached to it?

The answer is that they don't slide, rather the whole nail bed and nail grows out together. In the same way that the skin of your inner ear (all the way to the outer ear canal) grows outward, but it's not as though it's "sliding" anywhere. If you could look at a slice of your finger and recognize individual cell types, you'd see that the cells of the nail bed grow up, and out. The whole process is naturally carried forward, but as a unit, not with a sliding motion. Does that make sense?

In a brown or white cow(A scoop or two of ice cream in a cup that is then filled with soda), why do the carbonated bubbles remain for so long?

Oh, an ice cream float!! The dissolved milky fat in the ice cream increases the surface tension in the bubbles, which means they don't pop as easily/quickly.

Is it possible (physically, not financially) to build a telescope large enough to resolve stars in other galaxies?

Hubble can resolve stars in the Andromeda galaxy.

Are there any effects of returning transplanted organs to living donors?

Feasible - yes. In practice its never done because the removal will lead to some ischemic damage, living donors are specifically selected for their ability to live a healthy life with only one kidney, and the risk of adverse events during implantation is significant.

Does gasoline expand when colder?

No, it's the opposite. As temperature increases, the molecules in a liquid push against each other more and more, slightly increasing the volume. It's not huge, but you get the best value in gasoline if you buy early in the morning. > Why does water expand anyways in colder temperatures? When water freezes, it expands. This is because water is shaped like /'\ . What it's a liquid, all those V shapes jumble about randomly. When it freezes, they form neat /'\ \,/ shapes, with each end of the v hooking into each other. These crystalline structures take up a bit more space than V's jammed into each other, so volume increases. However, if water freezes at much lower temperatures or higher pressures, it can form less compact forms of ice that don't take up as much space.

Does gas actually get cooler when it expands in vacuum?

What you're describing is called a free expansion of the gas, which is an irreversible process. You're correct in saying the internal energy stays constant, but U is only a function of T only for ideal gases. So for ideal gases free expansion is isothermal, but real gases do change in temperature. In free expansion you have an entropy increase even though there's no heat exchange, because the process is irreversible.

Do planets still orbit stars that are densely-packed near the galactic core?

As /u/W00ster said, this is hard to observe. We rely a lot more on computer simulations or theoretical predictions. From this we infer that in those dense regions the orbits of planets are not stable, most of them would get ejected after a hundred million years (for comparison, the solar system is 50 times older). I don't have specific sources about the galactic core, but [this paper](_URL_0_) describes the stability of planets in globular clusters, which are supposed to be the cores of ancient galaxies that were later merged into the Milky Way.

[Physics] If matter can't move faster than light, how did the inflationary epoch of the big bang make the universe much bigger than one lightsecond in radius in one second?

Matter can't move through the coordinates of spacetime with a velocity of c, but that speed limit does not apply to the changing geometry of spacetime itself. Inflation is not the spreading of matter through space, rather it's the early expansion of spacetime itself.

What chemical reaction causes food to go stale?

In the case of bread, it's starch crystallization as /u/steinbergergppro mentioned. In the case of fats (which go rancid), it's oxidation by air.

Because there is "virtually" no friction in space, wouldn't you be able to reach the speed of light and surpass it eventually if you keep accelerating?

Whilst friction slows down your acceleration, you do intrinsically need energy to move in the first place, even with no friction, based on how much mass you have and how fast you're going. Special Relativity states that the amount of energy you need increases exponentially to infinity as you approach the speed of light - unless you have no mass (like light itself!), in which case you can only travel at the speed of light.

What's in my urine that makes it foam when it hits the toilet water?

I believe it is protein and amino acids. This phenomenon is used in marine aquaria to treat the water _URL_0_

Is it possible to build an optical telescope that could give you clear images of exoplanets?

This would require interferometry on a massive scale that we don't yet have the technology to implement (but it's not impossible). To see in the the optical band with 1000 km resolution from 100 lightyears away (just spitballing parameters here), you'd need a telescope about 5000 km wide. This is not feasible, but an array of telescopes could do the job (the Very Long Baseline Array already does this for radio astronomy). This array of telescopes could either span the surface of the Earth (while being close enough to point at the same thing), or could be in space.

Are there seasons on the moon or on the other planets in our solar system?

Yes. Every planet has seasons to some extent. For Mars, see here: _URL_0_ The moon is almost perfectly lined up in its orbit, the tilt is less than two degrees. Coupled with the lack of atmosphere, the Moon doesn't have seasons that mean anything. There needs to be an atmosphere for even larger tilts to have meaning, which is limiting for some planets. On the extreme end, Uranus has a tilt of 98 degrees. So it has seasons that are far more extreme than anywhere else, with 42 years of sun at a pole, dark on the other. Google can get you details on this. Most planets can be said to have four seasons, but its important to note that we humans picked four points to call seasons for biological reasons, not cosmological ones.

What causes involuntary twitching/jerking movements when falling asleep?

It's called a Hypnic jerk. The exact cause is unknown. The consensus among researchers is that as you drift off and your muscles relax your brain misinterprets this as falling. Your brain then sends signals to arm and leg muscles to regain balance. This would also explain the falling sensation some people experience just as they drift off. Hypnic jerks fall under the broad category of Myoclonic jerks.

Do planet orbits lie on the same plane?

The eight remaining planets (sorry pluto) orbit in *approximately* the same plane. The reason being that they formed from the [Protoplanetary disc](_URL_0_) that formed around the young sun billions of years ago. In general, clouds of gas collapsing under gravity tend to form discs, because of conservation of momentum/angular momentum. Pluto was always the odd man out, being offset from the earth's orbit by about 17 degrees, and this was actually a contributing factor in its reclassification.

Why are there zero-calorie sodas but not, say, zero-calorie cupcakes?

erm, because carbohydrates have calories. Not sure how you make the cake without flour.

My math teacher in 5th grade told us this formula to find out if a number is prime. I still use it today, but never found it mentioned/proved in mainstream books. Anyone know if it works for all prime numbers?

This doesn't work, but it's interesting to examine why it appears to work for a few cases more closely. x^2 - 1 = (x + 1)(x - 1) If x is prime, and x does not equal 2, then (x + 1) and (x - 1) are both even numbers. Let (x - 1) = 2k, then the test is that if k(k+1)/6 is a whole number, then x=2k+1 is prime. Obviously, either k or k+1 is an even number, but k cannot be odd, as then 2k+1 would be an even number and therefore not prime. Then k is an even number, and k+1 is odd. Let k=2j. Then the test becomes: if j(2j+1)/3 is a whole number, then 4j +1 is prime. Obviously, exactly one of (x-1)=4j, x=4j+1, (x+1)=4j+2 is divisible by 3, and (x-1) and (x+1) are both even. So what the test actually shows is that if (x^2 - 1)/24 is a whole number, then x is not divisible by 2 or 3. I doubt that it is coincidental that this tests the first two primes, so I assume that somewhere along the line someone misunderstood a method of methodically checking divisibility by 2,3,5,7,11,... to see if a number was prime.

Are there any predispositions to Alzheimer's? Is it genetic? Would a genetic basis mean that the diseases is also hereditary?

There are a number of genes that have been found to be associated with increased risk of developing Alzheimer's disease. The first one discovered was the ApoE4 allele, which increases risk by between 10 and 30 times. Others have recently been discovered which increase risk by smaller magnitudes, such as presenilin-1 and -2 and amyloid precursor protein (APP). Mostly these are associated with early-onset or familial forms of Alzheimers. Apart from the very rare familial form, it's not hereditary in the classical sense of the word - one of your parents having AD does not mean that you're going to get it. Quite frankly, we still don't know what causes it. It's therefore not entirely possible to determine whether or not it's genetic.

Why can high frequency (x-rays) and low frequency (radio waves) both travel through materials even though visible light cannot?

One way to understand how this can be possible: visible light can travel through solid materials as well (such as windows).

What makes ocean water bright blue in some parts of the world, but dark green in others?

There are different types of water throughout the Earth. For instance, the Great Lakes are a Type II water. This means it's has 3 particular color producing agents (CPAs). These are chlorophyll (providing blue and green colors) , dissolved organic matter (providing dark yellow/tan/brown color), and suspended minerals/sediment (providing a red to deep brown color depending on the sediment type). So, you will get water color based on the individual concentration of each of these CPAs. For ocean waters there may be different levels or types of CPA, but I know in coastal regions of the oceans the three i mentioned are common, esp chlorophyll. On mobile so I can't get sources, however I am a freshwater remote sensing professional. Edit: here is a book on remote sensing of coastal waters _URL_0_

If an old person donates an organ that is successfully transplanted to another person, will the organ live longer than it would have if it had stayed in it's original owner's body? If so, how is the life of the organ extended?

follow up question, could that organ be perpetually transplanted?

Do transplanted organs also replace themselves every 7 years?

Worth noting that the cells that replace those transplanted tissues are coming from stem cell reservoirs within those tissues, so they're still going to be "transplanted." It's not as if the tissue identity is going to slowly revert to the genotype of the transplant patient.

Are supermassive stars less dense than the sun?

You're correct. In general, the larger the radius of a star, the less dense it is. The densest normal stars are the M-dwarfs (the smallest possible stars). Obviously not counting dead stars like white dwarfs/neutron stars/black holes that are all way denser. Almost all stars with a radius larger than the Sun are going to be less dense than the Sun with very wispy atmospheres.

Could Earth plants survive on Mars?

Not without a lot of help. What has been found so far with Martian soil is that it's sterile and very salty. So you would need to introduce all the organic stuff that makes up soil here on earth (bacteria, rhizomes, fertilizers, water and so one) and flush out the excess salts. Before you even think about growing anything.

If standing in the sun for too long causes us skin cancer by damaging our DNA, why doesn’t the same thing happen to plants?

Plants produce their own ‘sunscreen’ which consists of phenols. They have a protein called UVR8 which detects levels of UVB radiation and responds by increasing the sunscreen production. Interestingly different species of plants naturally produce different amounts of protective susncreen depending on where they originate from. Those from high altitudes/the tropics tend to produce more due to the higher levels of UVB there. This is similar to how people from Africa evolved to produce more melanin to protect themselves from the suns damage.

Why are oranges pre-sliced by nature?

Each "slice" of the orange is a separate carpel, the part of the flower which produces the ovule. Each carpel contains many ovules (what becomes the seed) and many other sugar-water filled cells.

If we run out of helium, can we get more to replace it?

Helium can be "produced" through nuclear fusion or radioactive decay. Here on earth it occurs when Uranium or other heavy elements give off an Alpha particle which then picks up an electron to become stable helium. From what I have read it doesn't sound as though "production" is an option. But some project to be a much higher helium reserve in the United States than what has actually been proven. Yes, we may in fact be up shit creek. Please see the occurrence production section _URL_0_

What is something that is planet-sized, but doesn't orbit a star?

Yup, they're referred to as "rogue planets." _URL_0_ Not much is known about them, they could be planets ejected from their parent stars or even objects that formed naturally without a parent star.

Why do tires deflate with cold air despite the temperature outside of the tire dropping as well?

This is due to the ideal gas law, PV=nRT, which establishes the relationship between pressure, volume, temperature, and number of moles in an ideal gas where P=pressure, V=volume, n is the number of moles, R is the ideal gas constant, and T=Temperature. Due to the relationship between T and P, assuming the other variables, V and R, remain constant (R is a constant by definition of about 8.3 J/(K*mol)) — as T decreases P will decrease and vice versa. So in your example, when it goes from warm to cold outside, the decrease in temperature will cause a decrease in pressure. An example using 1m^3 for V and 20 moles for n: At 25C, P will equal about 7.19 psi At 5C, P will equal about 6.71 psi. Note: Low temperatures do not imply a low pressure. When evaluating pressure, all variables must be taken into consideration. However, because we are assuming volume and number of moles will remain constant, we can ‘kind of’ ignore them.

Is it possible for an atom to have more than 8 valence electrons? If cannot, what is the reason for this?

Yes, it happens all the time with transition metals and you even see hypervalency with some main group elements that have d-orbitals. d-orbitals hold an additional 10 electrons. The 8 valence electron rule is a useful teaching tool for high school students to get a hang of chemistry but isn't strictly true for all atoms.

Why does wet paper tear so much more easily than dry paper?

Paper engineer here. Paper stays together due to hydrogen bonds between the fibers. Coatings can add to the strength, but let's just talk simple, uncoated paper for this. Adding water gives additional sites to bond to and leads to the dissociation of fibers from one another as bonds go from fiber-fiber to an unsteady equilibrium of fiber-fiber and fiber-water. Edit: After thinking about it, I want to clarify that what /u/budpickens says about lubrication isn't right. The dissociation is aided by mass convection. This why an agitated tank can break down paper much quicker. It looks kind of like lubrication, but concentration gradients are a huge factor.

If water is transparent, why are the clouds white?

The scattering described by /u/ahhwell is called [Mie Scattering](_URL_0_) which is the reason many powders and small droplets look white. In this months Physics World there is an article about bread, which mentions amongst other things that white bread is transparent when you look at the strands of gluten under a microscope. Only the bulk scattering gives the white appearance, for the same reason.

How much plastic gets into plastic-bottle vodka?

Polyethylene Terephthalate: Polyethylene Terephthalate (PET, PETE or polyester) is actually quite resistant to alcohol and oil. Pthalates and diethyl phthalate (DEP) are widely present illegally produced spirits/alcohol and there is some evidence of negative health risks, you would have to be drinking a lot. It's hard to type/research on my phone but here's the one I managed to go through briefly _URL_0_

Nanoseconds after the big bang, wouldn't time be near-infinitely slow due to the energy/mass of the entire universe in a very small area?

No, gravitational time dilation requires a gradient in the potential, i.e. you have to compare two clocks where one clock is much further down in the gravitational potential than the other clock. After the big bang, the Universe (and thus the potential) was extremely homogeneous. Why it expanded exponentially is still up to debate. It would solve a lot of problems if it did, that's why we believe it did. But the mechanics are not entirely understood yet. It might involve some scalar field tunneling from a false to a true vacuum state.

Is there any evidence of moons having their own moons anywhere in the galaxy?

There is as yet no evidence for planets having moons anywhere outside the Solar System, much less moons around moons. Measurements are just now getting [sensitive enough](_URL_0_) that the detection of moons outside our the Solar System should be possible. As for the challenge of a moon having a moon, take a look at [this](_URL_2_) or [this](_URL_1_). Bottom line, it's not logically impossible, but having a stable configuration would be unlikely.

Why is it easier to balance a bike while moving ?

We don't know! There was a [paper in Science](_URL_3_) just this year on some of the newest results. It used to be that everyone thought it was the gyroscopic stability from the spinning wheels. But then somebody constructed a bike that had counter-spinning wheels to negate that, and it turned out it could be ridden anyway. It's not likely to be any single thing. But it's a (somewhat) active area of research, actually.

My question is: how do fingerprints form?

The skin contains 2 different sections that are called the epidermis and dermis. The epidermis is the outer section (closer to the surface) and the dermis sits below it. The layer of the epidermis which is in contact with the dermis is called the basal layer, this is the layer important to fingerprint formation. Before the fingerprints form the cells in the basal layer all sit in a straight line. For reasons that aren’t exactly clear, around the 10th-16th week of pregnancy the cells of this layer begin to become undulated (take on a wavy arrangement instead of a straight line). As time progresses this will change the arrangement of the layers of the epidermis above it. Eventually these undulations appear on the surface of the epidermis too where they are seen as fingerprints. This is also the reason that superficial skin injuries do not destroy your fingerprints. They are caused by something that is deep underneath the skin so damage to the surface doesn’t prevent them from re-forming when the skin heals.

Why do I always sneeze twice?

Just a friendly reminder as this thread has a potential to explode into a large discussion. Please follow the [AskScience community guideline](_URL_0_) as well as the reminder next to the submit button, avoid making top-level anecdotes, jokes, and layman speculations. Thank you for your cooperation and let's keep AskScience awesome.

Are all elements produced in stars?

No. Or at least there are some elements that we cannot possibly detect even if they were produced in stars. There are some elements which have extremely short half-lives. Which means that they can be created in laboratories, and there's a very short time within which these can be detected before they decay into other elements. That means that even if they are made within stars or through chance chemical reactions, we wouldn't be able to detect them ( considering the distance and the very short time period they would exist) Earlier all transuranic elements were thought to be like this, so that they could only be produced synthetically. But now we know that there are those that exist naturally as well (like Plutonium). Those with atomic numbers above 99 have only been created in laboratories and have not been generated in stars. I think also Technetium (Tc 43) is not present in nature, but not sure about that one.

Are trees monophyletic, or has "treeness" evolved independently in multiple plant lineages?

Multiple times. There are ferns that are [trees](_URL_0_), grasses that are [trees](_URL_1_), gymnosperms (conifers and ginkos), and many different lineages with trees within the flowering plants.

In images of distant galaxy, what are the things around it that look like stars? Are they actually just massive stars?

They are stars in our galaxy, much closer than the galaxy in the image.

Why is the ozone layer thinnest around Antarctica, why is ozone depletion not evenly spread around the world?

From wikipedia: _URL_0_ > Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to many scientists. There is a temperature difference between the north and south poles that accounts for the difference in the depletion of each.

Do plants and animals share a common ancestor?

[Here is a good summary](_URL_0_) Plants and animals share a lot of structure at the cellular level. Cells have chromosomes, ribosomes, phospholipid bilayers for cell membranes, they all use DNA to store genetic information, various forms of RNA are common to both plants and animals, etc.

Do plants and animals share a common ancestor?

Short answer: Yes! Long answer: Actually plant life isn't that different from animal life when you look at it on a molecular basis. As plants are eukaryonts (their cells have a nucleus) they are even more closely related to us than for example bacteria. Their DNA resembles ours in many ways and even codes for some of the same proteins. This is most probably due to a common ancestry. Concerning the origin of life... there is no evidence that life on earth started more than once, all living things most certainly share a common ancestor and lines split only after evolution kicked in (as evolution could only start AFTER life had begun).

Does a horizontally moving mirror reflect any differently than an equivalently stationary one?

I can think of at least one thing that would affect the reflection off a mirror moving at high lateral speeds, though there are probably more. There is some (very short) time delay between when the mirror absorbs incident light and when the mirror re-emits the the incident light. This is because whenever light falls onto the surface of a conductor, it doesn't just interact with the surface, it interacts with all the atoms near the surface to a depth roughly equal to the skin depth of the metal, which is usually a few nanometers, or a few hundred atomic radii. This gives the reflected beam a very tiny lag, of something like 10^-18 seconds. This means the reflection would probably be dragged along the direction of motion by a very small amount.