Why do apples turn brown?

Apples and other produce (e.g., pears, bananas, peaches, potatoes) contain an enzyme (called polyphenol oxidase or tyrosinase) that reacts with oxygen and iron-containing phenols that are also found in the apple. The oxidation reaction basically forms a sort of rust on the surface of the fruit. You see the browning when the fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals.

The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or another acid), reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide). On the other hand, using cutlery that has some corrosion (as is seen with lower quality steel knives) can increase the rate and amount of the browning by making more iron salts available for the reaction.

Why are zebras striped?

As a child I was always full of questions. I remember asking my parents why zebras were striped or why did giraffes have such a long neck. Most of the time the answers were elusive and I used to be very irritated. I could never get the right answer to satisfy my curiosity. I now realize why my parents could not give me a concrete answer. You see I happened to be in the same dilemma when my three-year-old daughter asked me the same question! I was faced with the difficult task of giving her half-baked answers or finding the right answers this time.

The most obvious question to ask about zebras is why are zebras striped? Unfortunately nobody really knows the answer. Looking at zebras in the zoo, the striped pattern seems very conspicuous and could hardly be thought of as protective.

Zoologists believe the stripes on a zebra could be one of several reasons. It could be basically for camouflage very much like the military fatigues. The bold wavy lines of a zebra blend in with the tall wavy grassy plains of Africa where these animals live. The bold stripe may even serve to break up the shape of the Zebra. If a zebra is standing still in such surroundings, a lion, its chief predator, may overlook it completely.

It doesn't matter that the zebra's stripes are black and white and the grass are dusty brown or green, because the lion is colour blind!

However this may benefit an individual zebra in some situations but is it likely a large zebra herd would be able to escape a lion's notice? This is exactly where it helps the zebra most. Zebra herds are very large but they stay very close to one another.

When all the zebras stick together in a herd, the pattern of stripes blends in with the stripes of the zebras around it. This is confusing to the lion! All it can see is a large, moving, striped mass instead of many individual zebras. The lion has trouble picking out individual zebras and it's harder still for the lion to recognize which way each zebra is moving!

Each zebra pattern is unique like a fingerprint. Also like a fingerprint, the patter varies from zebra to zebra and no two zebras are exactly alike. What is truly amazing is that zebras can recognize other zebras from their body stripe! A mother can instantly spot her colt in a herd!

There are three species classified according to the variations in the arrangement of the stripes - the Imperial or Grevy's zebra, the Plains or Common zebra and the Mountain zebra - and several subspecies. The mountain zebra is the smallest of these averaging only four feet. It has silver-white stripes with black markings that extend to every part of the body except the stomach and the inner part of thighs. Plains zebras travel in large herds and are pale yellow with broad black stripes. This specie has several variations: some have stripes down to the hooves while the lower legs of the others are solid white without any stripes.

Do you know that people of Africa think of zebras as black animals with white stripes whereas people outside Africa consider them as white animals with black stripes! Black stripes or white, the fact remains that zebras are extremely unique and are among the fastest and most graceful of runners in the African bush land.

Why are veins blue?

When someone asks the question "why are veins blue?" a likely response is that they're blue because the blood in veins is deoxygenated. While it's true that venous blood vessels carry a lower concentration of oxygen than their arterial counterparts, this isn't the reason for their blue appearance in your skin. Still, when someone invariably responds to the veins-are-blue-because-they're-deoxygenated argument with the observation that "I've never seen blue blood before" one might then hear the slightly more sophisticated-sounding but increasingly far-fetched claim that we don't ever observe blue blood because it is immediately oxidized upon contact with air.

Wrong. Blood is never blue. Ever. Period. (And, this is an excellent example of why you should never ingest information from Yahoo Answers uncritically.)

The bright red color of arterial blood stems from a complex that's formed between hemoglobin, iron, and molecular oxygen. This complex mainly absorbs higher energy (shorter wavelength) blue and green light, leaving behind primarily just red wavelengths for our eyes to detect. However, even when blood is largely depleted of oxygen, it is never blue: it's more of a deep maroon color. And, although I have not participated in a surgery myself, those who have (including my girlfriend, a veterinary student) assure me that in surgery, veins within the body do not appear blue either. So, not only is blood not blue, veins on their own aren't either. "Blue veins", then, are a phenomenon unique to the skin.

So, what's going on here?

The most comprehensive answer that I've been able to find comes from a paper by Kienle et al. published over twelve years ago in the journal Applied Optics and entitled "Why do veins appear blue? A new look at an old question". Based on the findings of the authors, three reasons emerge for the blue appearance of veins in skin. The first two are physical and stem directly from the way in which light interacts with blood (how it is absorbed) and with skin (in this case, how light is reflected). The final reason is psychological, dealing with the way in which our brain processes information relatively to generate color perception. (Greg Laden sort of hinted at some of this recently on his blog).

Since the paper in question is a study in optics, it is way outside of my area of expertise. So, just in case you wanted an in depth technical critique of its methodology, you're not going to find it here. However, I can give you a basic summary of the paper's major findings. And then, if you still have the desire (and the ability) to explore the subject in greater depth, you can parse through the paper yourself.

To tackle this problem, the authors measured how much light of various wavelengths was reflected from both real blood vessels in skin and imitation vessels in a skin-like environment, using a sophisticated technique that gave them spatially-resolved measurements. The synthetic vessel (which was a capillary tube filled with blood and placed in a milky substance with optical properties similar to skin) allowed the authors to experiment with a variety of parameters (particularly vessel depth and diameter), and they were then able to validate their results by taking measurements on actual vessels in skin. Using this set-up, they were able to demonstrate that the optical properties of skin and blood (combined with the influence of relative color perception) explain why veins in skin appear blue, despite not actually being blue.

Skin does not absorb much light at any wavelength, making it look white (depending on how much melanin is present, of course--making this discussion only really relevant to people with lighter skin). Blood, on the other hand, absorbs light of all wavelengths (but less in the red part of the spectrum). However, blue light does not penetrate the skin as well as red light. If a vessel is near the surface of the skin, almost all blue light is absorbed by the vessel, so even though only about 1/4 of the red light is reflected, the ratio of red light reflected to blue light reflected is about 10:1. This vessel appears red.

If the vessel is deeper (about 0.5 mm or more), not as much blue or red light will be absorbed. Importantly, this effect will be more pronounced on blue light than on red light since blue light doesn't penetrate skin very well (the ratio of red light reflected to blue light reflected is about 3:2 or less). This is the case for the "blue veins" observed in skin. Once the vessel is deep enough, though, it won't be seen at all, as light of all wavelengths will be reflected before it can interact with the blood.

Perplexingly, this 0.5-mm-deep vessel appears blue despite reflecting slightly more red light than blue light. This is where relative color perception comes into play. The surrounding skin reflects more red light than blue light (by a ratio of about 5:3), and it does not absorb as much of either type of light as a blood vessel does. Since vision is influenced in part by relative perception, if something purple is placed next to something red, the purple object will appear blue.

None of this, however, addresses the question of why veins specifically appear blue. To answer this one, I can fortunately once again rely on the expertise of Meredith, my vet student girlfriend. The reason why only veins appear blue is that veins are the only vessels we actually observe through the skin. This is due to the fact that veins are larger, have thinner walls, and are more superficial than arteries (and, no, I don't meant that veins prefer People or Us Weekly over The New Yorker--"superficial" is just medical speak for closer to the surface). All of these aspects of veins have clear biological rationales. Beyond just carrying blood back to the heart, the primary function of the venous system is as a blood reservoir. In fact, about two-thirds of your blood volume is held in your veins at any given time, hence their larger size. Because the heart has to push blood directly through arteries, their walls are subject to higher pressures than the walls of veins, so they need to be thicker. Finally, veins are located closer to the surface of the skin, because they also play an important role in heat exchange with the outside environment (to help cool the body). Arteries could perform this function just as well, but it's much more advantageous to keep those higher pressure blood conduits deeper in the body and protected from injury.

The take-home message here is that the bluish appearance of veins in the skin has everything to do with where they are located, and nothing to do with the concentration of oxygen within them. In fact, if we could see them through the skin as well, even arteries would look blue.

Why are sunsets red?

Sunsets often have a red or orange color to them. Why is this? Sunlight (what we call "white light") is made up of all different colors of light, each having a different wavelength. During a sunset, more red light is scattered toward you because of aerosols in the lower atmosphere, compared to the amount of blue or green light. Since, at sunset, sunlight is passing through a much longer path of the lower atmosphere than when the sun is overhead, the effect of the aerosols becomes much stronger. So, you end up seeing more red light that any of the other colors of light, and the sky appears red.

Why are some eggs brown?

White eggs come from white chickens and brown eggs come from brown-ish chickens. Most of the eggs in your supermarket come from the following breeds of chickens: the White Leghorn, the Rhode Island Red, the New Hampshire, and the Plymouth Rock.

White Leghorn chickens are white and lay white eggs. Rhode Island Red, New Hampshire and Plymouth Rock chickens are all reddish brown and lay brown or brown-speckled eggs.

Let's get weird for a second and pretend you have a chicken sitting beside you. Imagine this crazy chicken is kind of an off-white brownish yellow. You're no chicken expert and you have no idea what breed you're looking at. Here's the secret to predicting the color of eggs a chicken will lay: look at their earlobes. This is true stuff. The pigments in the outer layer of the eggshell will always approximate the color of the earlobe of the chicken that laid the egg.

A natural follow-up question would be "Is one color of egg healthier than the other?" According to the Egg Nutrition Center in Washington, D.C., the answer is a pretty firm "no". The color of the shell has nothing to do with egg quality, nutritional value or flavor. They say the reason brown eggs cost more is because the brown-egg variety of chickens are bigger eaters and cost more to feed. The cost is then pushed forward to the consumer. I happen to believe the real reason is that the health food industry is perpetuating the myth that brown eggs are healthier. There, I said it.

Why are lips red?

First of all, did you know that the outline or the border of your lips (called the vermillion border) is a special feature of humans only? This transition line from your skin to the pinkish-red part of your lips is found only in humans—no one knows why.

The lips appear red because of the underlying blood vessels. Arteries are blood vessels that carry blood back to the heart. The arteries and veins are connected through a series of blood vessels called the capillaries.

These red-colored blood filled capillaries are close to the thin skin on your lips, so your lips appear red.

Why are leaves green?

The green color in leaves is caused by the presence of a compound called "chlorophyll," which the plant produces to do photosynthesis, which is how plants get energy from sunlight. The purpose of chlorophyll is to assist the plant in making food for itself by utilizing energy from sunlight and nutrients from the soil.

The chlorophyll is green because it absorbs red and blue wavelengths of light -- and so green is not absorbed and is reflected, making the plant look green to your eyes. Non-transparent things are the color they are because that is the color that is NOT absorbed -- the color you see is the color that is reflected... which is why you see it!

Why are jeans blue?

Denim is unique in it's singular connection with one colour. The warp yarn is traditionally dyed with the blue pigment obtained from indigo dye. Until the introduction of synthetic dyes, at the end of the 19th century, indigo was the most significant natural dye known to mankind, linked with pratical fabrics and work clothing.

The durability of indigo as a colour and it's darkness of tone made it a good choice, when frequent washing was not possible. In 1870 BASF in Germany, originally suppliers of natural indigo had started the search for a synthetic substitute, in 1894 the process was perfected.

Why are flowers brightly colored?

To attract birds and insects.

Why are fire trucks red?

The most widely-accepted reason that fire engines are painted red dates back to the 1800s -- a time when there was a lot of competition between the fire brigades of neighboring cities and towns. The firefighters of each brigade took great pride in their pump. Each brigade wanted their rig stand out by being the cleanest, having the most brass, or being a regal color. Because red was the most expensive color, that's what color most crews chose to paint the pump.

Other sources cite the tradition of painting fire engines red going back to the early 1920's. Henry Ford wanted to make cars as inexpensively as possible and only offered cars in one color: black. With all of these black vehicles on the road, the fire service began painting their vehicles red in an effort to stand out.

Today, just as you have many more choices of colors available to you for your vehicle, so do the fire engine manufacturers, and it is not uncommon to see white, yellow, blue, orange, green, or even black fire engines, in addition to red. And while some studies hint that colors such as lime-green may be more visible to the public than traditional red, the vast majority of fire departments continue to use red fire engines -- a color instantly recognized by everyone as that of a fire engine.

Most recent fire engines purchased have shifted to the Chicago-famed, black over red paint scheme. The first closed-cab chief's cars in Chicago had black canvas tops which would not take paint. Someone among the brass liked the appearance, so as new closed-cab apparatus came onto the roster, the cabs of the fire engines were painted black.

You may also notice the green light on fire engines in northern states. This is also a traditional Chicago-style fire engine feature. Commissioner Albert Goodrich of the Chicago Fire Department (1927 - 1931) had a nautical background. He applied the marine scheme (red light on port, green light on starboard) to fire apparatus, and the idea became a tradition of the Chicago Fire Department. It is also used to mark the bay doors at most Chicago fire stations

Why are flamingos pink?

Flamingos are pink because of the food they eat, which is shrimp and algae.

Additional Input

It's not the the color of the food that makes the flamingo pink, but the chemicals inside.

Flamingos are pink if they have a lot of food containing beta carotene in their diet (i.e shrimp).

The coloring is actually a derivative of a class of plant-produced compounds called carotenoids-that is, carotene and its chemical cousins. Flamingoes in the wild eat shrimp, which is pink. The flamingo body may not process other colorful chemicals the way it processes carotene. Often times when flamingos are fed at a zoo, their diets are much different than their diet would be in the wild. Flamingoes in the wild eat shrimp, which is pink. Therefore flamingos in a zoo habitatand that aren't fed shrimp lose their pink color, so zookeepers must add food coloring to the food!

The depth and shade of their pink depends on how much suitable carotenoid they eat. In order to have flamingoes that remain pink in captivity, various colouring agents are fed to them; normally the same agents that are fed to farmed salmon so they have pink meat.

This answer is completely wrong! I'm pretty sure shrimps do not contain beta carotene. Beta carotene is an orange (not pink) pigment found in carrots and is a precursor to vitamin A. Whereas Astaxanthine is a pink pigment produced by plants and algae. It is Astaxanthine that give Flamingos their pink color. When shrimps eat algae the astaxanthine contained in them is incorporated into their chitin shell. When the Flamingos eat the shrimps the flamingos then incorporate the pigment into their feathers. Incidentally this is also why the meat or muscle of an adult wild salmon is pink. Farmed salmon raised on meal lacking the pigment have white appearing muscle. This is also why when you cook shrimp or lobster or crab they turn bright pink to red in color. Shrimps and lobsters don't look pink at first due to proteins in the shell obscuring the pigment. When the protein is denatured (cooked) it reveals the wonderful color. This is also true of the leaves of fall. When the green chlorophylls in the leaves are destroyed the reds and oranges and yellows of fall are revealed in their splendor! What about pink you say? I'll explain this next.

Both astaxanthine and beta carotene are Carotenoids, a group of pigment molecules produced by plants and algae that range in color from yellow to red (the leaves of fall). They help absorb light and pass the energy to chlorophyll. They can be divided into two groups, xanthophylls and carotenes. The former containing oxygen and the later lacking. The coloration of these molecules is due to the length of their carbon chain with alternating single and double bonds, called conjugation. This chain absorbs light of various colors (energies). The more highly conjugated or the longer the chain the more red the pigment looks. The shorter the chain the more yellow the pigment. Changing the length changes the light absorption spectrum and therefore the color. Vitamin A (retinol) alpha and beta carotene, lycopene, and lutien are all vitamins and antioxidants as well as Carotenoids.

Why are clouds white?

There are three primary colors: red, green, and blue. There are also three secondary colors that are created when you combine red and blue: violet. Red and green: yellow. Green and blue: cyan.

By combining all of the colors, you will get white.

In the same way that skies are blue because of scattering, clouds are white because their water droplets or ice crystals are large enough to scatter the light of the six wavelengths (red, yellow, green, cyan, blue, and violet), which combine to produce white light. White means all colors are present.

Color is light energy emitted in a way called the visible spectrum. It's visible but there are other forms that are not visible, like infrared or microwave energy.

The different levels of visible light energy have different colors because they have different wavelengths. If all colors of the visible spectrum are present, then the shade of light is white. If no light at all is seen, then the shade is black.

Why are barns red?

If you've ever driven through a rural area, it's likely that you've seen the red barns that speckle the farming landscape. There are several theories as to why barns are painted red.

Centuries ago, European farmers would seal the wood on their barns with an oil, often linseed oil -- a tawny-colored oil derived from the seed of the flax plant. They would paint their barns with a linseed-oil mixture, often consisting of additions such as milk and lime. The combination produced a long-lasting paint that dried and hardened quickly. (Today, linseed oil is sold in most home-improvement stores as a wood sealant). Now, where does the red come from?

In historically accurate terms, "barn red" is not the bright, fire-engine red that we often see today, but more of a burnt-orange red. As to how the oil mixture became traditionally red, there are two predominant theories:

Wealthy farmers added blood from a recent slaughter to the oil mixture. As the paint dried, it turned from a bright red to a darker, burnt red.

Farmers added ferrous oxide, otherwise known as rust, to the oil mixture. Rust was plentiful on farms and is a poison to many fungi, including mold and moss, which were known to grown on barns. These fungi would trap moisture in the wood, increasing decay.

Regardless of how the farmer tinted his paint, having a red barn became a fashionable thing. They were a sharp contrast to the traditional white farmhouse.
As European settlers crossed over to America, they brought with them the tradition of red barns. In the mid to late 1800s, as paints began to be produced with chemical pigments, red paint was the most inexpensive to buy. Red was the color of favor until whitewash became cheaper, at which point white barns began to spring up.

Today, the color of barns can vary, often depending on how the barns are used.

Why are Michael Jackson's kids white?

Because they aren't biologically his.

Why are most plants green?

Most plants appear green because they contain chlorophyll, a pigment within the leaf and stem structure that reflects the wavelengths of light that make up the color green. Chlorophyll absorbs other wavelengths of light and uses the energy to produce nutrients.

Why is the sky blue?

A clear cloudless daytime sky is blue because molecules in the air scatter blue light from the Sun more than they scatter red light. This scattering effect is known as Rayleigh scattering. Because the blue light is scattered in random directions, some of it reaches the planet surface, where we see it.

When we look towards the Sun at sunset, we see red and orange colors because the blue light has been scattered out (filtered) and away from our line of sight.
In the evening, the sky sometimes looks orange or red because of air pollution. dust, water vapor, and other floating particles in the air act as a filter on the sunlight. When the Sun is low, the air layer is thicker and the light is more filtered, so it looks yellow, orange and finally red.

In more detail:

Light of a particular color is characterized by its frequency and wavelenth. The higher the frequency, the more blue it appears.
Sunlight is made up of all colors that, when mixed together, produce white light. You may have seen a rainbow or the prism experiment where the white light is split up into several colors.

The earth's atmosphere is filled with minute dust and water particles that act like a filter, scattering the light rays. The rays of light with the longer wavelengths, such as reds and yellows, tend to pass more easily through the atmosphere, while the rays with the shorter wavelengths, like blues and indigos, tend to be randomly scattered more easily. These more easily dispersed shorter light rays are what give the sky its blue color.

Red skies at sunrise and sunset are caused by the same phenomenon. When the light hits the Earth at an angle it has more of the atmosphere to go through; this increases the filtering effect and that is why you see a red sky.

Why is my poop green?

Healthy people can have green poop if they eat a diet rich in leafy green vegetables, or if they consume large quantities of food coloring (in ice cream, cake frosting etc.).

Green poop can also be caused by excess iron in the diet, from dietary supplements, for example. If the body does not absorb all the iron consumed, the iron may stain the poop green, the color of iron (II) salts. Ordinarily, the green color may be masked by the normal brown poop color, but if digestion is thrown off by illness so that bilirubin is less concentrated in the intestine, the green color may become apparent. This can happen when a person is afflicted with diarrhea.

Green poop in sick babies may come from iron in baby formula not being properly absorbed, or by green pigments in bile salts (again, green from iron).