A calorie is a unit that measures how much energy your body can get from food. When you eat an apple, a sandwich, or a cookie, your body breaks down that food and converts it into energy, just like a car converts gasoline into the power to move. Scientists use calories to measure how much of that energy different foods contain.

Your body uses this energy for everything: running, thinking, growing, keeping warm, and even sleeping. A stick of celery might contain only 10 calories because it provides very little energy, while a slice of pizza might contain 300 calories because it provides much more fuel for your body.

Different activities burn different amounts of calories. Reading a book burns calories slowly, while playing soccer burns them much faster because your muscles are working harder. Growing kids need plenty of calories because their bodies are building new bone, muscle, and brain tissue.

Food labels list calories to help people understand what they're eating. If you see that a granola bar has 150 calories, that number tells you approximately how much energy your body will get from it.