A calorie is a way of measuring the energy in food and drink. We need energy so that we can move. Even our beating heart while we sleep needs energy. It all comes from food and drink.

We use up calories of food to do things. But the body does not know where its next food is coming from, so it stores away any surplus in case there is a shortage of food in the future. It stores it as fat. Some of the fat is inside us and you never see it. The rest is just below the skin. So, if we eat more calories than we need, this is stored as fat. That is why scientists have spent a lot of effort trying to find out how many calories we need. It is not obvious how many calories are in what food, so that is what we rely on scientists to tell us. But, as a rule of thumb, fruit and vegetables have few calories (but lots of other important things), sugar and dairy products and meat have most calories, and bread and cereals are in between.