If you watch television very much -- and most kids do -- you hear a lot about vitamins. You know that you need them to stay healthy. But have you ever wondered what vitamins are, or how they work in your body?

Doctors and scientists found out about these special substances a few centuries ago. Here's how it happened.

During the 1500s and 1600s, sailors who went on long sea voyages often fell ill with a mysterious sickness called scurvy. Their gums swelled up; their teeth fell out. If they got cuts or scratches, the wounds didn't heal. Many sailors died.

What was happening? Doctors were confused by the disease, because sailors on some ships didn't catch it. Sailors who had fresh fruit to eat during their voyages stayed healthy.

During the 1740s, a British doctor named James Lind decided to do an experiment to find out if the fruit was keeping the sailors from catching scurvy. He tested his idea on sailors who had scurvy. He divided them into teams, and gave each team a different thing to eat every day. Some of the men took a paste made out of spices; some of them drank small amounts of vinegar; some got small amounts of sea water. And some of them ate two oranges and one lemon every day.

The results were dramatic. The men who ate the citrus fruit -- the oranges and lemons -- got better within two weeks. The other men didn't. There must be something in the fruit that prevents scurvy, Lind thought.

Today, we know that the "secret ingredient" found in citrus fruit is a substance called vitamin C. The word "vitamin" means "chemical essential to life."

Scientists have identified 13 different vitamins. There may be others that haven't been discovered yet. They help you use the food that you eat to build a healthy body. Your body makes some of the vitamins itself, but the others have to be supplied by food.

Doctors say that the best way to get enough vitamins is to eat a healthy diet. Many people take vitamin pills to make sure that they get their daily supply of vitamins -- but these pills are not necessary for people who eat well.

Sometimes, people will come down with illnesses that result from a lack of certain vitamins -- as sailors did with scurvy. Then, a doctor may prescribe special vitamin pills to treat the disease. But for most people, healthy eating does the trick.

You've already learned that vitamin C prevents scurvy. But vitamin C has other important jobs, too. It keeps the tissues that form your body, including your bones and blood vessels, strong. It helps build the material that holds your cells together -- a kind of human cement called collagen.

So you must have vitamin C for strong bones and teeth, and healthy gums. You also need it to help heal up the scratches and cuts you get. And it helps your body fight off infection.

You already know that citrus fruit like oranges, grapefruit and lemons contain the vitamin. So do tomatoes, raw cabbage, potatoes, strawberries and cantaloupe. You can also get vitamin C by eating brussels sprouts, broccoli, green peppers, and collard or mustard greens.

Vitamin C is a tricky substance. It can disappear from food that has been overcooked, or has soaked too long in water. You can make sure the vitamins are still in your fruits and vegetables by eating them raw. If you want them cooked, ask your parents to steam them. Fewer vitamins get lost when food is steamed.

A good place to get your daily vitamin C is in a glass of orange juice. Have you ever tried combining your morning juice with your morning milk to make a breakfast shake? Here's how to make a pitcherful of orange shakes for your family. The recipe comes from "Creative Food Experiences for Children," a book by Mary T. Goodwin and Gerry Pollen.

You'll need a cup of orange juice, 1 1/2 cups of powdered milk, 3 cups of water, a few ice cubes, and a pitcher. First, mix powdered milk and the water in the pitcher. Add the orange juice and stir. Then add the ice cubes and drink up. If you have popsicle molds, you can pour the leftovers into the molds, and have orange popsicles for an afternoon snack.

You can be sure you're getting the vitamin C you need by eating an orange every day, or by drinking tomato, orange or grapefruit juice. It's just as easy as remembering to take a vitamin pill -- and the fruit or juice tastes a whole lot better. Tips for Parents

Dr. Alvin Mauer, professor of medicine and pediatrics at the University of Tennessee Center for the Health Sciences in Memphis, doesn't like the proliferation of vitamin advertising on television.

"It instills the concept that a trip to the corner drugstore can keep you healthy," he says. "There are lots of kids who think it's necessary to take pills to be well. I would like to have them think that you don't need pills to keep healthy, but rather that they're something you might need to take if you get ill."

Mauer also heads the American Academy of Pediatrics' Committee on Nutrition. According to the committee, "Normal children receiving a normal diet do not need vitamin supplementation over and above the recommended dietary allowances."

So the problem becomes getting your child to consume a balanced diet. Mary Goodwin, coauthor of "Creative Food Experiences for Children" and a nutritionist for Montgomery County, suggests involving your children in the planning, purchase and preparation of meals. "Don't worry them about vitamins; get them interested in food -- its smell, its feel, its color," she says.

The book contains a section on nutritious bag lunches. It is available from the Center for Science in the Public Interest, 1501 16th St. NW, Washington, D.C. 20036 (332-9110).