The Odd Body, Explained

1. What causes goose bumps?

Goose bumps (scientific name: piloerection) pop up when you're cold or afraid. A tiny muscle at the base of each body hair contracts; together, they appear as naked bumps on the flesh. They made sense eons ago, when humans still had a natural "fur coat." Back then, fluffing your ruff would warm the body by trapping an insulating layer of air between the hairs. And standing your hair on end was intimidating to predators or enemies (picture a cat facing off with a dog). Evolution has since stripped humans of their pelts. Now goose bumps are, of course, no medical issue. If you're uncomfortable showing off your vestigial physiognomy, dress warmly, place yourself in calm environments, and avoid horror flicks.

2. Why does chopping onions make you tear up?

When you cut into an onion, you rupture its cells, releasing enzymes that produce a gas called propanethial sulfoxide. Once that gas reaches your eyes, it reacts with tears to produce a mild sulfuric acid. And that hurts. The brain then signals the eyes' tear glands to produce more liquid to flush the stuff out. The more you chop, the more irritating gas you produce and the more tears you shed. "The onion's chemical reaction is a defense mechanism that evolved to repel pests," explains University of Wisconsin-Madison horticultural professor Irwin Goldman, Ph.D. Keep the stinging and crying to a minimum by chilling an onion in the freezer before cutting it; cold temperatures slow release of the enzymes. The highest concentration of enzymes is at the bottom of the onion, so cut it last to postpone the weeping (and the irritation) for as long as possible.

3. Why do your joints crack?

The most common type of joint in the human body is the diarthrodial joint—knuckles and shoulders are examples—in which two bones come together in a capsule. Inside that joint capsule is a lubricant called synovial fluid, which contains dissolved gases. When you stretch the joint, you're actually compressing it and the fluid within, forcing those nitrogen-rich gases to escape the synovial solution. The release of "air" within the joint capsule is what you hear as a "pop." Once the gas is released, the joint is a bit more flexible (enabling you to go a little further in a yoga pose, for example). But you've probably noticed that you can't immediately crack the same joint again. That's because the gases released in a pop must first reabsorb into the fluid, a process that takes 15 to 30 minutes. If you habitually crack your knuckles to relieve tension, try concentrating on your breath for 30 seconds instead. Knuckle cracking doesn't lead to arthritis, but it can lead to decreased grip strength.

4. When you find something really funny, why do you sometimes "laugh until you cry"?

Experts don't really know. One thing to consider: Laughing and crying are similar psychological reactions. "Both occur during states of high emotional arousal, involve lingering effects, and don't cleanly turn on and off," says Robert R. Provine, Ph.D., a psychologist at the University of Maryland, Baltimore County, and author of Laughter: A Scientific Investigation (Penguin, 2001). We associate crying with sadness, but tearing up is an even more complex human response. Tears are triggered by a variety of emotions—"by pain, sadness, and in some cases even extreme mirth. It's just the way we've evolved," says Lee Duffner, M.D., a professor of ophthalmology at the University of Miami's Bascom Palmer Eye Institute. As it turns out, that's good, because both laughter and crying can ease a stressful experience, probably by counteracting the effects of cortisol and adrenaline. So if you ever find yourself laughing until you cry, count yourself lucky.

5. What makes your eyelid twitch?

This annoyingly common condition is known as eyelid myokymia. Not a lot is known about eye twitches, which are more likely to occur in the lower eyelid than in the upper, though they're probably caused by the misfiring of a nerve. But experts know that fatigue, stress, and caffeine all increase the likelihood of the pesky twitching. So do eyestrain, poor nutrition, excessive alcohol intake, and allergies. Fortunately, eye twitching is almost always benign and usually goes away by itself. To put an end to a bout of the eye flutters, cut down on coffee and alcohol and give your eyes—and your whole body—a good night's rest.

6. Why do some people always feel cold?

Body temperature is regulated in the brain by the hypothalamus, which signals the body to give off heat in warm conditions and trap heat (or shiver, generating heat in muscles) when it's cold. Iron plays a role in this process, so people with anemia (commonly caused by iron deficiency) often feel chilly. Poor circulation—due to high blood pressure or medications, among other culprits—can leave the extremities deprived of heat. An underactive thyroid gland can also slow a person's metabolism to a point where the body generates insufficient warmth. A recent study suggested there may even be a genetic predisposition to toward tolerance of cold. If you're the type who needs to wear sweaters and wool socks in the summer, eat iron-rich foods like lean red meats, beans, and dark green leafy vegetables, which can counter anemia. And avoid nicotine, which constricts blood vessels and leads to poor circulation.