When people talk about the calories in food, what do they mean? A calorie is a unit of measurement — but it doesn't measure weight or length. A calorie is a unit of energy. When you hear something contains 100 calories, it's a way of describing how much energy your body could get from eating or drinking it.

Calories aren't bad for you. Your body needs calories for energy. But eating too many calories — and not burning enough of them off through activity — can lead to weight gain.

Most foods and drinks contain calories. Some foods, such as lettuce, contain few calories (1 cup of shredded lettuce has less than 10 calories). Other foods, like peanuts, contain a lot of calories (½ cup of peanuts has 427 calories).

You can find out how many calories are in a food by looking at the nutrition facts label. The label also will describe the components of the food — how many grams of carbohydrate, protein, and fat it contains.

Here's how many calories are in 1 gram of each:

carbohydrate — 4 calories
protein — 4 calories
fat — 9 calories

That means if you know how many grams of each one are in a food, you can calculate the total calories. You would multiply the number of grams by the number of calories in a gram of that food component. For example, if a serving of potato chips (about 20 chips) has 10 grams of fat, 90 calories are from fat. That's 10 grams x 9 calories per gram.

Some people watch their calories if they are trying to lose weight. Most kids don't need to do this, but all kids can benefit from eating a healthy, balanced diet that includes the right number of calories — not too many, not too few. But how do you know how many calories you need?

How Many Calories Do Kids Need?

Kids come in all sizes and each person's body burns energy (calories) at different rates, so there isn't one perfect number of calories that a kid should eat. But there is a recommended range for most school-age kids: 1,600 to 2,500 per day.

When they reach puberty, girls need more calories than before but they tend to need fewer calories than boys. As boys enter puberty, they will need as many as 2,500 to 3,000 calories per day. But whether they are girls or boys, kids who are active and move around a lot will need more calories than kids who don't.

Most kids don't have to worry about not getting enough calories because the body — and feelings of hunger — help regulate how many calories a person eats. But kids with certain medical problems may need to make sure they eat enough calories. Kids with cystic fibrosis, for instance, have to eat high-calorie foods because their bodies have trouble absorbing the nutrients and energy from food.

Kids who are overweight might have to make sure they don't eat too many calories. (Only your doctor can say if you are overweight, so check with him or her if you're concerned. And never go on a diet without talking to your doctor!)

If you eat more calories than your body needs, the leftover calories are converted to fat. Too much fat can lead to health problems. Often, kids who are overweight can start by avoiding high-calorie foods, such as sugary sodas, candy, and fast food, and by eating a healthy, balanced diet. Exercising and playing are really important, too, because activity burns calories.

How the Body Uses Calories

Some people mistakenly believe they have to burn off all the calories they eat or they will gain weight. This isn't true. Your body needs some calories just to operate — to keep your heart beating and your lungs breathing. As a kid, your body also needs calories from a variety of foods to grow and develop. And you burn off some calories without even thinking about it — by walking your dog or making your bed.

But it is a great idea to play and be active for at least 1 hour and up to several hours a day. That means time spent playing sports, just running around outside, or riding your bike. It all adds up. Being active every day keeps your body strong and can help you maintain a healthy weight.

Watching TV and playing video games won't burn many calories at all, which is why you should try to limit those activities to 1 to 2 hours per day. A person burns only about 1 calorie per minute while watching TV, about the same as sleeping!