Do you know the story of Tom Sawyer? If so, you probably remember that he and his friend Huckleberry Finn tried out smoking one afternoon. Puffing on pipes made them feel pretty grown up -- but it also made them pretty sick.

Mark Twain wrote the story of Tom Sawyer about 100 years ago. Since then, scientists have learned a great deal about just how sick smoking can make people. Even so, some kids still think that smoking is a cool thing to do.

IT ISN'T! In fact, smoking can kill.

To find out why you shouldn't smoke, you need to understand how your respiratory system -- the part of your body that does your breathing -- works. Let's follow a breath as it moves from the atmosphere around you, down into your lungs and back out again.

First the air enters your body through your nose and mouth. Hairs inside your nostril catch particles of dirt that may be in the air. A wet, sticky substance called mucus lines your nasal passages. It captures some of the pollution, germs and dust in the air.

Your respiratory system, just like your nose, is coated with a sticky mucus lining. In it, tiny hair-like bumps called cilia constantly sway back and forth like a field of wheat in the wind. They are so small that they can only be seen with a microscope. You have millions of cilia.

Their job is to sweep away germs and dirt. They do this by waving back and forth about 12 times per second. They sweep the bad things toward your mouth and nose, where they can be gotten rid of. When you cough or sneeze, you get rid of these trapped particles.

From your mouth and nose, the breath of air you inhaled -- which contains an invisible gas called oxygen -- continues to your lungs. If it is cold air, it begins to warm up as it moves through the respiratory system. It soon reaches the same temperature as your body -- about 98.6 degrees.

From your mouth and nasal passages, the breath swooshes into your throat, and down your windpipe, or trachea. From there, the breath flows into a system of tubes called the bronchial tree. The bronchial tree gives shape to your lungs, two football-sized organs inside your chest. You can feel your lungs getting bigger and smaller when you breathe in and out.

Wait a minute -- a tree inside your lungs?

It's not a real tree, of course, but a branching system of passages that looks like an upside-down tree without any leaves. Like branches and twigs, the air passages in your bronchial tree gradually get smaller and smaller. At the tips of the smallest parts are small clusters of sacs called alveoli. They have a very important job to do.

From the alveoli, oxygen enters your blood. Red blood cells pick it up. Your heart pumps the blood through your body, carrying the oxygen to every single cell from the top of your head to the tips of your toes. That vital oxygen keeps you alive.

When they pick up oxygen at the alveoli, your red cells leave behind carbon dioxide, a gas left over after your cells have used up oxygen. This harmful gas leaves the body when you breathe out, or exhale.

So why shouldn't you mix tobacco smoke in with the air you breathe? There are lots of reasons.

Tobacco smoke contains many harmful things. It contains a chemical called nicotine. When nicotine enters your body, it makes your blood vessels get smaller. Your heart has to work harder to pump your blood. Tars in cigarette smoke coat the inside of your nice pink lungs, staining them brown.

Substances in the cigarette smoke cause cancer, a very serious disease. Today, doctors know that smoking is the leading cause of lung cancer, which kills many people every year.

Tobacco smoke also contains carbon monoxide gas. When this gas reaches your alveoli, it passes into your red blood cells. This prevents them from picking up their load of oxygen -- so a smaller amount of this important gas reaches your cells.

Cigarette smoke is also very hot. It rushes into your lungs fast, before your respiratory system can cool it off. The heat irritates the sensitive lining of your bronchial tree.

Remember those cilia that work so hard to seep away harmful particles in the air you breathe? Well, just one cigarette slows the action of those tiny hairs. If someone smokes for a long time, the cilia become paralyzed. They stop moving. Very heavy smoking can even destroy the cilia. When cilia stop working well, more germs enter the lungs, leading to more illnesses.

Today, one out of every four Americans is a smoker. Maybe one of your parents smokes. If so, you probably already tried to convince them to stop. You may have said, "Please don't smoke, you're making yourself sick."

If there is cigarette smoke in the air around you, you inhale it even if you don't light up yourself. Studies done by medical researchers have shown that the lungs of kids aged 5 to 9 who live with parents who smoke don't work quite as well as the lungs of kids who live in smoke-free houses. Children who live with smokers have higher heart rates, higher blood pressure, and more frequent illnesses, too.

The American Lung Association reports that a million American teen-agers start smoking every year. Their advice to you? DON'T START! If you never smoke, you can be breathing easy for a long, long time. Tips for Parents

An anti-smoking poster featuring Singer Stacy Lattisaw is available from the D.C. Lung Association for $2. Branches of the American Lung Assocation also conduct programs to help adults stop smoking. To enroll, call 682-LUNG.