Did you know that most smokers first try cigarettes between the ages of 12 and 14? That's what a study conducted by the National Institute on Drug Abuse reports.

Wait a minute . . . what's an institute on drug abuse doing worrying about cigarettes? There's a good reason. The tobacco in cigarettes contains nicotine, which is a strong drug. And like many other drugs, nicotine is addictive. That means people who use it can come to depend on it and need to use it to feel normal.

Like other drugs, nicotine causes changes in your body. When people smoke, they breathe in tar and harmful gases such as carbon monoxide in addition to nicotine. These substances stop your lungs from working as well as they should.

Here's what happens when your lungs are doing their job right: When you inhale, you breathe in a lungful of air. Air contains oxygen, a gas your body requires as fuel for all your cells. The oxygen gets into your body through your lungs. Inside your lungs is a complicated system of passages that get smaller and smaller. At the ends of the smallest passages are little air sacs called alveoli. Oxygen passes through these little sacs and gets into your bloodstream. From there, your red blood cells deliver the oxygen all over your body.

The cells use the oxygen for energy. When your red blood cells pick up oxygen from your alveoli, they unload another gas called carbon dioxide. This gas is the waste product your cells make as they do their jobs. When you exhale, you release this gas into the air.

But when you mix tobacco smoke in with the air you breathe, you cause a whole bunch of problems. Among them: The nicotine in the smoke makes your blood vessels constrict, or get narrower. That means your heart has to pump harder to get the blood through -- and that's bad for your heart. A gas called carbon monoxide in cigarette smoke interferes with the exchange of oxygen and carbon dioxide that happens in your alveoli. Red blood cells pick up carbon monoxide in place of oxygen. That means your cells get less oxygen. Once again, the heart beats faster to deliver more blood to make up the difference. And that's not good for the heart. Tars in the cigarette smoke stick to the moist, spongy lining of your lungs. So do other harmful substances in the smoke. These substances can cause the killer disease lung cancer. Smoking is the leading cause of this cancer. Hot cigarette smoke burns and irritates the delicate lining of your lungs. More germs get into your lungs when you smoke. That's because smoke paralyzes the cilia that line your breathing system. Cilia are tiny, hairlike bristles that work all the time to sweep germs and particles of dirt out of your lungs. When the cilia slow down or stop, more gunk gets in, and you're more likely to get colds and respiratory infections. Just one cigarette slows cilia down; lots of cigarettes stop cilia from moving altogether.

Because of these effects, cigarettes are very, very bad for your health. They lead to lung problems that make it hard to breathe, such as chronic bronchitis and emphysema. They help cause heart disease and heart attacks. And they cause cancer.

In spite of these dangers, experts estimate that about 20 percent of American teenagers smoke cigarettes. And they're concerned about how the habit is going to affect their health in the future. Antonia Novello, the Surgeon General of the United States, is very worried about how many cigarettes kids smoke. Last month, she said that more than 3,000 teenagers become regular smokers every day. If that keeps up, she estimates, 5 million of today's kids will end up dying from smoking-related illnesses when they are adults. She thinks that the government should make the prevention of smoking among kids its No. 1 health concern.

The Surgeon General and other government officials would like to see cigarette machines banned, and they would like to make it illegal for kids under age 19 to smoke. Right now, most states have laws restricting the sale of cigarettes to minors. But those laws aren't enforced very strictly, Novello says.

Many people all over the country are trying to stamp out cigarette smoking. The American Cancer Society, the American Lung Association and the American Heart Association are all working toward that goal. They want to help people who already smoke quit, and they want to prevent kids from ever trying cigarettes.

When someone offers you a cigarette, what will you decide?Tips for Parents

Parents are the strongest influence on whether children smoke, says the American Academy of Pediatrics. If both parents smoke, a child is twice as likely to become a smoker than if neither parent smokes. The best way to stop your kids from smoking is to quit yourself. Contact your local chapter of the American Lung Association for help. While you're at it, ask them about Lung Association publications for children, among them "Don't You Dare Breathe that Air," "As You Live and Breathe" and "The Smokeword Puzzle."

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