Calories are a way of measuring energy – the amount of energy that your body gets from eating that food. Your body needs a certain amount of energy every day to keep it going - things like keeping your muscles moving, your heart beating, your body heated and your brain ticking. The energy comes from the food and drink that you consume. If you eat and drink just the right amount (with the right amount of calories in them) to move your muscles, work your brain etc then you won't get over or underweight. Eating too many calories in your food turns it into fat; eat too few and you begin to disappear.