Food labels contain the number of calories per serving. But how is this number determined? The answer is surprisingly simple: The food is burned. A sample of the food is placed in an insulated, oxygen-filled chamber that is surrounded by water. This chamber is called a bomb calorimeter. The sample is burned completely. The heat from the burning increases the temperature of the water, which is measured and which indicates the number of calories in the food. For example, if water temperature increases by 20 degrees, the food contains 20 calories. This method of measuring calories is called direct calorimetry.