Now that December has arrived, you may find you're shivering and shaking as you wait for the bus or walk to school in the morning. Brrrrrr!

When it's very cold outside, the heat from our bodies starts to escape into the air around us. When the outside of the body gets cold -- like when you've been waiting on a windy sidewalk for what seems like hours -- your muscles start squeezing together and then relaxing again. They do this over and over again, and do it fast. We call this movement shivering, and it creates extra heat to keep your body warm.

You've probably noticed that shivering is something that just happens. You don't decide to do it with your mind; your body decides for itself. That kind of automatic response is called a reflex.

But how does your body know when to start shivering? The answer has to do with a part of you called the hypothalamus.

"The hypo-what?" you say.

Before we get to that, let's look at something more familiar: skin. If you look closely, you'll see that it's covered with tiny holes called pores. When you get hot, the pores open up and sweat comes out of them. As the sweat evaporates, your body cools off. During winter, pores close up tight, shutting warmth inside, sort of like shutting a bedroom window on a freezing night.

How do your pores know when to open and when to close? That's where your hypothalamus comes in. Your skin contains very sensitive nerves that can tell the difference between hot and cold. When your nerves sense cold, they flash a message to the hypothalamus, which is located in your brain. The hypothalamus has a vital job: It regulates the temperature of your blood.

We human beings are warm-blooded animals. The cells that form our bodies produce heat. Most of the time, unless we get sick, we stay at a temperature of about 98.6 degrees Fahrenheit. At that "normal" temperature, our inner organs -- the heart, lungs, kidneys, liver and so on -- all work well. The blood that circulates through the body keeps everything at that nice, even temperature.

But if your blood starts to cool off, the hypothalamus goes into action. It sends signals racing through your body. Your blood vessels tighten. If it's very cold outside, your body automatically cuts down on the flow of blood to your extremities -- parts that are far from the heart, like your hands, feet and face. That explains why your hands, feet, ears and the tip of your nose are the first things to get cold when you spend a long time outside in the winter.

As your skin cools, the hypothalamus sends another message to your muscles. They begin to move -- and you warm up by shivering and shaking.

Your body has to work hard to stay warm when it's cold outside. You can help by dressing correctly for cold weather and by going inside if the weather is dangerously cold. Here are some tips from the American Academy of Pediatrics:

Always wear a hat. It's true that you lose a lot of heat through the top of your head. A hat warms the blood moving through your head and helps the body keep itself warm.

Dress in layers. Layers of loose clothing trap warm air around your body. The layer of warm air helps maintain your body temperature. Long underwear and turtleneck sweaters work well. Mittens are warmer than gloves because the fingers keep each other warm. Tight gloves can cut off blood circulation in the fingers.

Don't get wet. As water on the skin evaporates, it cools the blood below the surface of the skin -- just the opposite of what you want.

Cover bare skin. If it's very cold and windy, putting a scarf across your nose and cheeks will protect them. The scarf also protects you from wind burn -- irritation of the skin caused by the wind.

If you're so cold that it hurts, go inside. Pain is your body's warning signal.

If you start to feel numb, it's even more important to get inside fast. You could be getting frostbite, an injury that can actually make your skin and the tissues underneath it freeze solid.

Wear boots when you play in the snow. Toes are more prone to frostbite than other parts of the body because feet are often directly in the snow.

Between your own good sense and your hypothalamus, you should be able to have a comfortable winter.

Tips for Parents

The American Academy of Pediatrics, an organization of 39,000 children's physicians, provides these suggestions for winter safety:

Keep children indoors if the wind-chill factor is minus 20 degrees or below. Supervise outdoor play on very cold days. Children become colder sooner than adults -- but may be enjoying themselves too much to admit it.

Signs of frostbite include: skin that is gray or yellow in color, sometimes with white spots, and feels soft and cold to the touch. The child may report tingling. To treat superficial frostbite, bring the child indoors. Gradually warm the skin with warm (not hot) water, warm compresses or by placing the frostbitten area next to skin that is of normal temperature. Do not rub the affected area. In cases of severe frostbite -- the skin feels numb and is cold, waxy and pale -- take the child to the emergency room immediately.

Catherine O'Neill is a children's writer.