Have you ever seen a cat when it's out in the cold? It fluffs its fur up into a thick coat that protects the animal from the cold. When you feel chilly and get goose bumps, your hair stands up, too. But your "coat" isn't nearly as snuggly and warm as the one the cat wears.

You and that cat have something in common. You are both members of a group of animals called mammals. All mammals -- from huge whales to tiny mice -- have hair. Whales have only a few bristles around their mouths, but most other mammals are almost completely covered with hair.

Take a look at your arms, your legs, or your face. You'll notice that tiny, short hairs coat your skin. Human beings have hair everywhere but the soles of the feet, the palms of the hands, and the lips. Some people have a lot; others have only a little.

Scientists think that long, long ago our ancestors had much thicker hair than we have today. They needed it to keep warm. We don't have much of that furry coat left.

Hair isn't completely useless, though. On your head, it helps keep you warm during cold weather. In hot weather, hair shields your scalp from the sun.

Your eyebrows catch sweat and prevent it from running into your eyes when you exercise. Like natural sun visors, they cut down the amount of light that glares into your eyes on a sunny day.

Your eyelashes keep bits of dust and bugs from getting into your eyes. Eyelashes are also very sensitive. If something touches them, they send signals that alert your eyes to danger. Your eyelids blink shut.

Have you ever wondered why it doesn't hurt to get your hair cut, but it does hurt to get your hair pulled? The shaft of each hair -- the part you can see -- is made up of dead cells. When you cut through them, they don't send a pain message to your brain. But the roots of your hair are alive. When someone pulls on your hair, the roots flash out a message and you say, "Ouch!"

A tough substance called keratin forms your hair. Each hair grows as new cells form at its root. The cells push outward, building a shaft. Eventually, the hair dies and falls out. But a new hair grows in to replace it.

Keratin also forms your tough fingernails and toenails. Animals' claws are made of keratin, too. So are horses' hooves. Your nails aren't as tough as hooves, but they do protect the ends of your fingers and toes. The part you can see is made of dead cells, so it doesn't hurt to clip your nails -- just as it doesn't hurt to cut your hair.

When people feel horrified or frightened they might say, "My hair is standing on end!" Have you ever felt that way? Here's how it happens: Each hair on your body comes equipped with a tiny muscle that makes it stick upright. This erector muscle gets a message from your nerves when you are especially scared, angry or excited. Similar nerve signals flash to your erector muscles when you feel cold.

Remember the fluffy cat in the cold? The hairs that form its fluffy coat have erector muscles, too. Cats, dogs and most other mammals also have these muscles. When dogs growl at each other, their erector muscles make the hairs on the backs of their necks stand on end. Some wild animals fluff up their coats to look larger and fiercer when they meet an enemy.

When you feel scared or threatened, and your hair stands on end, your body is responding much as an animal's does -- but instead of looking fierce, you just look bumpy. Maybe that's why people use their hair mainly as a decoration nowadays, instead of depending on it to scare away enemies. Tips for Parents

Studies show that 50 percent of all school-age youngsters are nail-biters. The good news is that most kids grow out of it.

Dr. Catherine DeAngelis, deputy director of pediatrics at Johns Hopkins Hospital in Baltimore, says most health professionals believe nail-biting is a sign of tension, stress and nervousness. It frequently appears in children involved in a difficult home situation.

Nail-biting is not particularly harmful, although it cam lead to hangnails or minor skin disorders. Treat it lightly. Resorting to ridicule, bitter nail coatings or restraining devices may make the habit worse or lead children to replace it with something more serious, DeAngelis says. Try a reasoned appeal to your child's pride in his or her appearance -- but don't nag.

Catherine O'Neill is an editor of National Geographic World, a magazine for children 8 and up.