Why do we get goose bumps, go wrinkly in the bathtub, and sprout hair in unlikely places? We asked experts to explain our most curious quirks.

Why do I wake up in the morning with gunk in my eyes?
That crusty stuff is actually just dried tears. Your tear glands are constantly watering your eyes to flush out irritants. “While you’re awake, the act of blinking wipes tears away before they can build up,” says Jason Edward Karo, M.D., an ophthalmologist in Charlotte, N.C. “But when your eyes are closed at night, the dried tears accumulate inside the corners.”

Why do I get goose bumps during scary movies?
Goose bumps are produced by piloerection, a reflex that contracts the muscles around the base of each hair follicle, causing the hair to stand up (and small bumps to emerge), says Zoe Draelos, M.D., a consulting professor of dermatology at Duke University. Back when humans were hairier, piloerection had two benefits: It helped keep us warm by trapping heated air close to the skin, and in scary situations, it made us look bigger and more threatening (like a spooked cat puffing up its fur). Watching a horror flick in an air-conditioned theater provides the perfect setup for piloerection: You’re not only frightened, you’re also probably freezing.

Why do my fingers and toes wrinkle in the water?
Here’s the leading hypo-thesis: The top layer of your skin absorbs more water than the layers beneath, increasing its surface area, says Richard Scher, M.D., professor emeritus of dermatology at Columbia University. Since the newly baggy skin is still attached to the non-bloated layers, it bunches up, causing a prunelike effect. Fingers and toes are especially prone because of their thicker shell of dead keratin, an absorbent protein on the skin’s outer layer.

MosquitosWhy does it seem as if mosquitoes find me more delectable than other people?
It doesn’t just seem that way. The bloodsuckers are drawn to a tasty dinner by a variety of signals, including heat, carbon dioxide, movement, and the smell of skin ­secretions like lactic acid. One study even found that mosquitoes tend to prefer people who have recently downed a beer. No one knows what precise combination mosquitoes find the most irresistible, but if you seem to be their meal of choice, wear ­repellent and pass on the brew.

Runny NoseWhy does my nose run when it’s cold outside?
Your nose helps perform a kind of climate control by heating and humidifying the air that you inhale so that it better matches the moist, warm conditions inside your lungs. Glands in your nose produce secretions that add moisture, and blood vessels in your nose dilate to warm the incoming air, acting like miniature radiators. When you breathe ­super-frigid air, those ­phenomena ­are amplified, ­leading to runniness, says Andrew P. Lane, ­M.D., director of the Johns ­Hopkins Sinus Center. There are two reasons why: Cold air tends to be drier, which causes the glands in your nose to ­produce more secretions. Second, when you exhale warm, moist air out into the cold world, some of the ­moisture condenses into droplets of water, which collect at the tip of the nose and add to the drip.

Why can’t I stop biting my nails?
Casual nibbling may have evolved as a type of self-grooming, like when an animal bites or scrapes down its claws, says Monnica Williams, Ph.D., assistant professor of psychology at the University of Louisville. It may also be a soothing ritual for when you’re stressed, similar to sucking your thumb. But some people take this habit too far, gnawing away until their nail beds are painful or bleeding. If you’re ­overchomping, says ­Williams, seek the help of a doctor or cognitive behavioral therapist to help you break the habit.

Why do men get more hair in some places and less in others as they get older?
The “George Costanza effect” is a common sign of aging. It’s caused by dihydrotestosterone (or DHT), a sex hormone to which some men are ­genetically predisposed to become sensitive, according to Spencer Kobren, founder of the American Hair Loss Association. When that sensitivity develops, the hormone often causes the hair follicles on the head to shrink and ­follicles elsewhere, like on the back or in the ears or nasal passages, to ­become stimulated.

What are hiccups, exactly, and how can I stop them?
A hiccup is an involuntary contraction of the diaphragm and the muscles between your ribs. Any short-term hiccupping is called a “bout” and is ­usually the result of overeating or drinking carbonated beverages (other causes include sudden excitement, stress, or too much alcohol). Traditional remedies for bouts include holding your breath, sipping cold water, gargling, swallowing a ­teaspoon of dry sugar, ­gently pressing on your eyeballs, or leaning forward to ­compress your chest. If your hiccups last more than 48 hours, see a ­physician. An underlying medical problem, such as gastritis (inflammation of the ­stomach lining), might be causing them.

OnionsWhy do I cry when I chop onions?
According to Harold McGee, author of On Food and Cooking: The Science and Lore of the Kitchen, these tears are caused by the release of a chemical called a lacrimator (which sounds like a supervillain but is actually from the Latin verb “to cry”). When you slice into an onion, the chemical is expelled into the air, and once it reaches your eyes and nose, it breaks down into irritating compounds that attack the nerve endings there, says McGee. Your eyes produce tears (and, likewise, your nose may run) to rinse away these compounds. To reduce the effect, try prechilling onions in ice water for 30 to 60 ­minutes; this will slow the enzymatic reaction that releases the lacrimator, and that means fewer of the offending molecules circulating in the air and fewer tears.

What causes bad breath?
Typically, the culprit is your tongue—or, to be more specific, the white film that builds up on your tongue while you sleep, says Ken Yaegaki, Ph.D., D.D.S., head of the Nippon University Department of Oral Health. Yaegaki estimates that 60 percent of halitosis cases can be attributed to bad-smelling sulfuric compounds produced by this layer of microorganisms and dead cells. The solution: Try ­regularly brushing your tongue, back to front, using a soft-bristled toothbrush.

Why do I lose so much hair when I shampoo?
Most people lose 100 to 150 strands each day; the longer the strands, the more dramatic their appearance when you find them on your brush or in the drain. (And if you don’t brush or wash your hair every day, the loss will seem greater on days when you do.) If you think you’re losing an excessive amount—you’re no longer able to style your hair as you normally would, for example—then see your doctor. You’re most likely developing run-of-the-mill pattern baldness (which can affect both men and women), but there could be an underlying medical condition, like lupus or a thyroid disorder, at play.

Why do I huff and puff climbing stairs when I can easily run a mile on a treadmill?
As anyone who’s used a rolling suitcase knows, it’s a lot easier to pull a heavy object along a flat surface than it is to pick one up. When you run on a treadmill, you’re barely lifting any of your body weight up and down. Walking up a typical 45-degree staircase, on the other hand, requires you to move 70 percent of your body weight against gravity, says Joseph Signorile, Ph.D., professor of exercise physiology at the University of Miami and author of Bending the Aging Curve.

TicklingWhy is it impossible to tickle yourself?
The answer lies within the brain’s cerebellum, which helps to monitor body movement. Scientists from University College in London found that the cerebellum can predict how selfadministered touches will feel and alerts other tickle-sensitive areas of the brain. Since a truly successful tickle requires an element of surprise, this early-warning system makes self-tickling an exercise in futility.

What is earwax? And why do we need it?
Earwax is made up of moisturizing oils, infection-fighting enzymes, and dead skin, says Jordan S. ­Josephson, M.D., an ear, nose, and throat specialist at Lenox Hill Hospital in New York City. And though the substance may be unsavory, he argues that most of the time, we should let our earwax be. “When you try to remove it using Q-tips or your finger, the result is like packing a musket: You keep pushing the earwax in deeper, where it gets stuck and hardens,” says Josephson. Instead, clean your ears while showering by gently tugging the lobes, which straightens the ear canal and allows water to wash away excess wax.

Why do fingernails grow faster than toenails?
We know that fingernails grow about three times as fast as toenails and that the fingernails of your dominant hand grow more rapidly than those of your nondominant hand, says Nia Terezakis, M.D., clinical professor of dermatology at Tulane School of Medicine. What we don’t know for sure is why. The most ­accepted theory is that since you use your fingers more than your toes (and your strong hand more than your weak one), the faster nail growth might result from the body’s ­attempt to repair whichever parts are undergoing the most “trauma” (from activities like pecking out texts or digging for car keys).

Why do people ache more as they get older?
The older you are, the more likely you are to develop arthritis. The Arthritis Foundation estimates that 27 million Americans suffer from the most common form, osteoarthritis, which is caused when cartilage—the connective tissue that cushions our joints—­begins to break down. Without this protective padding, bones can rub together, causing stiffness and pain. Older adults are also more likely to develop a condition called bursitis, in which the bursa—the fluid-filled sacs that reduce friction where tendons slide over bones—become inflamed. The good news? Increasing your activity level may make the symptoms milder (and help you feel better).

Even More Of Your Body Explained!

Check out answers to questions that were too racy to print!

Why do some men get erectile dysfunction while others don’t?
There are several reasons erectile dysfunction (ED) can occur, says Paul Turek, M.D., a urologist and director of the Turek Clinic in San Francisco. In young adult men, the culprit is often stress. The reason? When you’re stressed, your “fight or flight” nervous system becomes activated (the same system that would kick in when your remote ancestors were, say, fighting off predators). When that happens, non-essential systems like digestion—or sexual function—get “turned off” by the body. Which makes sense if you think about it: Who wants an erection while being chased by a wooly mammoth? In men over 50, ED is typically caused by hardening of the arteries, which can impede blood flow to the genitals. This also explains why men with ED have higher rates of heart disease and heart attacks as they age compared with men without erection problems.

Why do some people experience incontinence as they get older?
The process for urinating is complex: It involves a “pump”—the bladder—and a “pipe”—the urethra. Connecting the two is a “valve” that controls the flow. As people age, any of these parts can become worn out, says Turek. The bladder and the urethra are both essentially muscles that can lose strength. In addition, conditions that affect the nervous system, such as Parkinson’s disease, a stroke, or diabetes, may interfere with the signals that connect the three parts, putting the whole system on the fritz.

Why do men get erections during sleep?
A typical man has at least three erections that each last up to one hour while asleep. When the body is alert, the tiny blood vessels in the penis are tightly contracted, but when you’re sleeping, the blood vessels dilate, allowing the organ to fill up with oxygen-rich blood. These nighttime erections are not only normal, says Turek, they keep the male reproductive system healthy—think of it as the body’s way of performing a “test run” when things are not in use.

Why do we have pubic hair (and armpit hair)?
We appear to be the only species of primate (perhaps the only species, period) that bears this type of hair, and scientists are still debating its purpose, according to Jesse Bering, author of the upcoming book Why Is the Penis Shaped Like That? Experts have speculated that it’s a signal of sexual maturation, since the hair tends to sprout up during early adolescence. Both pubic and armpit hair may also serve to trap pheromones—chemicals that some experts believe are excreted by the body to attract potential mates. It’s also not clear why pubic hair is so distinctly thick, short, and curly—though Bering notes that it’s likely a matter of practicality. To have long, flowing locks growing down there wouldn’t be terribly convenient given the logistics of sexual intercourse.

Why do we get flatulence?
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), most people pass gas anywhere from 14 to 23 times a day. Where does all that air come from? Some of it is swallowed—everyone takes in a bit of air while eating and drinking. The rest—between one and four pints a day!—is produced by the body as foods are broken down in the large intestine. Certain foods tend to produce more gas than others during the digestion process; these include foods high in sugar (soft drinks, candy, fruit), fiber (beans, whole grains), and starch (potatoes, pasta, wheat).

—Alex McDaniel

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