A calorie is a unit of energy—the amount of energy required to raise the temperature of one gram of water by one degree Celsius. Practically speaking, calories measure the amount of energy we get from food and burn through physical activity and metabolic processes such as breathing. (Also, since true calories are actually a minuscule measure, the familiar term "calorie," when used on food packaging and the like, actually refers to kilocalories.)
If you consume more calories than your body needs, your body stores the extra energy as fat. If you consume fewer than the needed number of calories, your body will burn fat to provide itself with energy. To determine how many calories you or your child needs each day, you can use an online calorie calculator (you will need to know the gender, age, and activity level of the person whose needs you are analyzing).
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