Body Mysteries 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. 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.

5. Is it true that your ears grow throughout life?

Yes, the outer ears do. Starting at birth, the ears are, proportionally, the body's largest feature, with a Spock-like prominence. They grow rapidly until about age 10, then slow to the languid pace of about 0.22 millimeter per year, according to a study by Britain's Royal College of General Practitioners. Other studies show that the earlobe itself also lengthens throughout life (men have longer lobes than women). However, the size of the ear canal, which is formed by bone and cartilage, does not increase into old age.

6. What causes the feeling of "pins and needles"?

Called paresthesia, pins and needles are caused by blocked blood flow to a pressed nerve. If you sit too long in an awkward position—or even just with your legs crossed—you may press hard enough on a nerve to interrupt its signaling to the brain, causing your feet, for example, to "fall asleep," or go numb. This is not the same as a pinched nerve, a longer-lasting condition that occurs when a part of the body, swollen because of injury or misalignment, applies steady pressure on a nerve. Paresthesia is usually felt in the extremities—hands, feet, and ankles. That crazy-making prickly sensation is the resumption of pain messages to the brain. Simply changing your position is almost always enough to allow the nerve to resume communication. But prickly feelings more rarely can be symptoms of diseases as diverse and serious as diabetes, lupus, and MS. If your pins and needles don't resolve quickly with a change of body position, see a doctor.

7. Why are there rings or halos around lights when you drive at night?

This phenomenon falls under the category of "spherical aberration"—just one of several examples of how the human eye is optically imperfect. In daylight, the pupil narrows to a very small opening, allowing light to hit the very center of the lens. At night, when the pupil dilates dramatically to allow maximum light to enter, your eye is using a much larger swath of its lens to see. "The farther out on the lens you go, the less perfect the optics are," says Duffner. "And as you get off center, those light rays won't be focused to the center of the eye." You see circles, well, because your lens is round. Almost everyone sees these rings, and if you've always seen them, you're probably just fine, he says. But halos can also be caused by opacities in the lens—a sign of cataracts. So if seeing halos is new to you, see a doctor for a cataract exam.

Taken from MSN.com