A calorie (say: cal-er-ee) is a measure of how much energy the nutrients protein, carbohydrate, and fat can supply your body. When you eat food, your body uses the food as fuel, burning it to produce energy, or calories. Every person's body needs energy to function. Whether your body is doing things you don't even think about, like pumping blood or blinking, or doing things you plan, like walking a dog or blowing bubbles, you always need calories!
Some types of nutrients have more calories than others. When you eat foods with protein, every gram of protein gives you four calories (a gram weighs about the same as the amount of sugar that's in a packet at a restaurant). Same deal with carbohydrates: when you eat foods with carbohydrates, your body burns the food as fuel and gives you four calories for every gram of carbohydrate. And fat gets its reputation for being fattening because when your body burns it, you end up with nine calories for every gram of fat! That's more than twice the amount!
1 cup of milk has 8 grams of protein, so:
8 grams X 4 calories = 32 calories from protein (that's 32 calories for you from the animal who says moo!)
1/2 cup of cooked spaghetti has 17 grams of carbohydrate, so:
17 grams X 4 calories = 68 calories from carbohydrate (use your noodle and eat spaghetti for lots of energy!)
1 order of "large fries" from your favorite fast-food restaurant has 22 grams of fat, so:
22 grams X 9 calories = 198 calories from fat (uh-oh, what's that? Lots of calories from fat!)
So if it's energy your body is after, you might think that the large fries would be best, since they give you so many calories, right? The answer is. . .nope! That's because your body can only use so many calories at one time. Whatever calories your body doesn't burn for fuel, it stores in your body as fat. So even though they taste good, foods that get a lot of their calories from fat aren't quite as great as they seem. . .in fact, if you eat a lot of them, you can end up gaining weight.
The key to keeping your body healthy is to remember that your body uses food for energy. When you are active, your body burns lots of calories to let you kick that ball far or run fast. But when you are watching TV or a movie, your body doesn't burn nearly as many calories - you just don't need that much energy to laugh at your favorite show!
So the trick to keeping your body at a weight that's good for your height ie; weight loss is to eat as many calories as you burn off. If you eat more calories and exercise less, the energy has nowhere to go, and you gain weight. The more you exercise, the more calories you can have, because there is a good balance between the two. But don't forget, it's still a good idea to get most of your calories from protein and carbohydrates. Fat won't hurt you if you eat it in moderation (moderation means eating it a little bit, and not very often), and it tastes good, but fat should make up a little less than 1/3 of all the calories you eat.