Have you heard about calories? Calories are a measure of the energy which food contains. A kilocalorie (kcal) is 1000 calories and is the commonest unit for measuring how much food energy you eat and how much food energy you use each day. If you eat as many kilocalories as you burn off when you take exercise, then you are eating just right. If you eat more than you burn off, you put on weight: you get fat. Okay, imagine filling the gas tank of a car. But our imaginary car's gas tank is special: it's like a balloon and the more gasoline you pump in, the bigger it gets, just as your tummy does when you over-eat. That's okay if you then drive your car on a long journey so it 'gets exercise'. The gas tank gets smaller and emptier as all the gasoline energy is burned up. And so it is with people. If you eat too much energy-rich food and don't burn it off, you get fat! Simple really.