Did you know that you have something in common with your dog or cat? You both have fur, you both have claws . . . Wait a minute! That's not right at all. Your cat or dog has fur and claws; you have hair, fingernails and toenails. What you have in common is the fact that those features are made of the same material. It's tough stuff called keratin that's manufactured in your skin.

Hairs are cylinders made of three layers of keratin. The outer layer has tiny overlapping scales that can be seen through a microscope. Fingernails and toenails are plates made of long, tough layers of transparent keratin.

Hair and nails are really specialized parts of your skin, and, like the skin, they protect your body from the outside world. Eyebrows help catch dirt and germs that might get in your eyes, keep sweat from running into your eyes when you exercise and help shade your eyes from the sun. Eyelashes also act as a barrier against dirt and germs. They're attached to sensitive nerves that act as a warning system for your eyes. If something touches one of your eyelashes, your eyes blink shut.

The hair on your head protects your scalp from the sun and helps keep you warm in the wintertime. Fingernails and toenails are shields for the sensitive tips of your fingers and toes. (Can you imagine wearing a pair of closed shoes if you didn't have any toenails? Ouch.)

The visible part of hair and nails is made of dead cells. That's why a haircut or a nail trim won't hurt. But nails and hair both grow from living roots that are hidden in your skin. If you pull your hair, the living root will object. If you bang your fingernail near the base, you'll find that there's a living root in there!

Here's how hair grows: Cells multiply and are pushed up from the live root hidden away in a deep layer of your skin. As they pile up, they form a long cylinder that continues growing at a rate of about five inches a year. Eventually, the base of the hair dies and the hair falls out. Later, that same root grows a new hair, and the cycle continues.

Hair grows on almost every surface of your body except on the palms of your hands, the soles of your feet and on your lips. The hairs can be so fine that they're practically invisible.

In cold weather, body hairs can be easier to see because tiny muscles next to each hair make the shafts stand up away from your skin. If you were a furry animal, this would help keep you warm by trapping a layer of insulating air next to your skin. Animals' skin and hair respond to cold the same way ours do. But they get warm; we only get goose bumps.

Unlike hairs, nails grow continuously. If they get very long, nails start to bend and curl like the horns on a ram's head. To keep them from getting so long that they break off, we trim them or file them. (In case you're wondering, fingernails grow four times faster than toenails.)

Like hairs, fingernails have living roots. They're tucked under the skin at the base of the nail. The nails grow out over the nail bed, a pad of tissue that has a very rich supply of blood vessels. Nails get their pink color from the blood showing through.

If you injure a nail by slamming it in a car door or hitting it with a hammer, it'll turn black from the blood that was spilled underneath it. Eventually, the nail may fall out. But as long as the root is intact, you'll grow a whole new one.

The way your hair and nails look can give doctors clues to your internal health. During a physical exam, your doctor will check your nails to see their color and shape. Certain illnesses give nails a bluish look or an odd shape. Streaks of white in the nails can be a sign of poisoning or illness. Dull-looking hair can be a sign that your body lacks Vitamin B12.

We spend a lot of time worrying about our hair and nails. We cut them, dye them, paint them, curl them and generally fool around with them. For us, hair and nails are a form of decoration, a way of telling others something about the kind of people we are. That has been true for a long time; people have been dying their hair, wearing wigs and painting their fingernails for centuries.

What do your hair and nails say about you?

Tips for Parents

A survey of 4,000 Kansas kindergarten through third-grade children showed that 42 percent of them exhibited "negative stress behavior," such as sleep disturbance, headaches, worries about doing poorly in school -- and nail biting. Antidotes to stress, says Darrel Lang, director of the Center of Health Promotion at Emporia State University, include relaxation exercises such as deep breathing, a proper diet, increased family communication and physical exercise. He says that kids who attended schools where they could blow off steam in physical education classes showed fewer signs of stress. So if your child starts nail biting, it might be worthwhile to make an effort to reduce any stresses you perceive in his or her life.

Catherine O'Neill is a freelance children's writer.