It's Thanksgiving again! Day after tomorrow almost everybody in America will sit down to eat a festive holiday meal. Let's face it. Day after tomorrow nearly everybody in America will sit down and stuff themselves -- and feel lucky and thankful to be doing it. At the beginning of the millions of Thanksgiving dinners, people will pause to remember the many things they have to be grateful for: families, homes, enough food to eat.

Then, they'll dig in to turkey, ham, stuffing, gravy, mashed potatoes, yams, beans, salad and three kinds of pie. After the meal, someone -- maybe Great Aunt Hildegarde or Cousin Arthur -- is sure to lean back and say something like, "I sure didn't need all those calories. But dinner was so delicious . . . could you pass me one more tiny piece of pumpkin pie?"

Have you ever wondered what a calorie actually is? People talk about them all the time. Advertisements on TV boast that a soda has "just one calorie." Doctors advise overweight patients to go on a reduced-calorie diet, while pregnant women are advised to add extra calories to their daily meals. People often make calories sound like they're something that's bad for you. They aren't. In fact, you need a certain amount of calories every day just to stay alive.

But you can't just go and buy a box of calories at your local store. They're in the food you eat, but they're not food themselves.

In fact, calories are a unit of measurement, the way inches are a unit of measurement for length, or pounds are a unit of measurement for weight. Calories are the unit of measurement for the amount of energy that's locked in the food you eat. Once the calories get into your body, you burn them up as you think, run, breathe and do all the other activities of daily life.

How is that energy measured? It's calculated as units of heat. In the most technical terms, a calorie is the amount of heat you'd need to raise 1,000 grams of water 1 degree Centigrade at sea level. Luckily, you don't need to do that to figure out how many calories are in your food. You can read the label or look the food up in a cookbook or on a calorie chart.

Some people talk about "good" calories and "bad" calories. In fact, all calories are the same. But some foods contain more calories than others. Fatty foods are high in calories. That's because a serving of fat contains more than twice as many calories as the same amount of protein or carbohydrates does.

You could probably eat three bowls of plain strawberries and still end up consuming fewer calories than if you had one bowl with a generous serving of ice cream on it. That's because the plain strawberries are just carbohydrates. The ice cream contains fat, with its double helping of calories. You get the idea.

Cutting down on fat can make a big difference in the total number of calories you eat every day. But don't cut fat out entirely; you need fat to do vital jobs like carry vitamins in your bloodstream.

You also need some fat in your body to protect your internal organs and keep your body temperature constant. Most nutritionists recommend that one third or less of a person's daily calories should come from fat. You get plenty of the fat you need from meat, eggs, fish, cheese, vegetable oil and other healthy foods.

People become overweight when they eat more calories than they burn up. The extra calories turn into fat. So the trick is to eat enough calories to stay well and keep growing, but not so many that you get fat. For most kids, this simply means following a well-balanced diet and getting regular exercise.

Remember, growing bodies need more calories than adult ones do. So don't put yourself on a low-calorie diet unless your doctor tells you that you need to. You could disturb your normal rate of growth if you do. Tips for Parents

A recent issue of Pediatrics reported that several children fed what their parents considered "ideal" diets were actually dangerously underweight -- to the point at which they stopped growing. The problem was simple: not enough food. Their parents were well-intentioned and were watching fat and salt intake and not allowing the children to eat between-meal snacks. It turned out that the children were receiving only 63 to 94 percent of the daily calories required for their age group. After nutrition counseling, the parents fed the children what they required, and the youngsters began to grow again. The moral of the story? "Moderation" should be the watchword in any dietary plan. Catherine O'Neill is a free-lance children's writer based in Baltimore.