What makes our body “get up and go” every day? Just like gas fuels our car, the calories we get from food fuel our bodies. Have you ever tried to explain how this works to your child? Let’s start with a definition of a calorie. A calorie is a unit of energy or heat which approximates the energy needed to increase the temperature of 1 gram of water by 1° Celsius. In simpler terms, a calorie is the energy we receive from food.