It's a chilly, windy morning in January. You bundle up, and set out from your nice, warm house to walk to school. By the time you get there, your body is shivering and your teeth are chattering. Brrrrrr!

Is something wrong with you?

No. In fact, your body is doing you a favor by shaking and shivering like a leaf in the wind. You may feel cold, but the movement in your muscles is actually keeping you warm.

As a human being, you're a warm-blooded animal. Your body is made up of tiny blocks called cells. And the cells use the food you eat to produce heat. When you're well, you stay more or less the same temperature. In most people, normal is 98.6 degrees Fahrenheit.

But when the surface of your body gets cold -- say, when you're outside sledding for an hour or so -- you start to shiver. Your muscles are contracting, or squeezing together, and then relaxing again. They do it very quickly. As the muscles contract and relax, they produce extra heat. That keeps your body warm.

What happens when you shiver?

Look closely at your skin. You can see that you're covered with tiny holes called pores. When you feel hot, these pores open up. Sweat pours out of them. As it evaporates, your body cools off. But during the winter, the pores close up tight -- shutting warmth inside. It's almost like shutting your bedroom window on a freezing night.

Your skin covers your body like a perfectly fitted glove. The outer layer -- the part you can see -- is called the epidermis. Under that layer is another part of the skin called the dermis. Your dermis contains blood vessels and nerve cells. The nerves in your dermis make you very sensitive. They allow you to feel the difference between hot and cold.

When your nerves sense the cold, they carry a message to a small part of your brain called the hypothalamus.

The hypothalamus keeps track of the temperature of your blood. When your blood starts to cool, the hypothalamus goes into action. It sends signals racing through your body. Your blood vessels tighten. Less blood goes to the surface of your body, where it can lose heat quickly. Your skin cools off -- but the important parts inside you stay warm.

Sometimes, the hypothalamus sends messages to your muscles. They begin to move -- and you warm up by shivering and shaking.

You don't have to think about how cold you feel to start those signals from the hypothalamus going. Your body does the thinking for you. That kind of automatic response is called a reflex.

Your body has to work hard to stay warm when the air around it is very cold. You can help by dressing in the right kind of clothes when you go outside in freezing weather.

You know that your toes and fingers can get stiff and numb when you're too cold. They start to hurt. The pain and numbness are signals from your nerves, too. They're sending you a message -- saying "HELP!" You can keep your skin from getting injured by covering it well to protect it from the cold environment.

Put on warm socks, mittens and a hat. Wear sweaters under a sturdy coat. The clothes keep your body's heat from escaping into the cold air. This kind of protection is called insulation.

Warm-blooded animals all have hair as insulation. Dogs and cats have thick, furry coats to keep the heat their bodies produce inside. When the temperature drops, muscles on the animal's skin tighten up. The hair stands on end and looks fluffy. The fluffed-up fur provides more insulation to keep the animal warm.

Human beings also have a "coat" of hair. But yours is too thin to do very much good in the cold. In cold weather, a human being's hair stands on end, too. That's when you get goosebumps -- another signal that your body is trying to keep you warm and healthy.

You can rely on your body to let you know how it feels. But to stay healthy, you need to pay attention to its messages. One doctor gives his advice:"Uncomfortableness is a signal. You were given those nerves to tell you something. Listen to your signals -- enjoy the cold weather, but don't stay out until your mittens are frozen solid and your hands and feet are numb."