Two kinds of calories exist, actually. Ask a chemist and you will learn that a calorie (with a lowercase “c”) is the amount of energy (heat) required to raise 1 gram (1 millileter) of water by 1°C. A nutritionist, on the other hand, would describe a “big C” or kilocalorie (kcal) as the amount of energy required to raise 1 kilogram (1 liter) of water by 1°C. The kcal is the unit used to describe the energy value in food. For example, if a chocolate chip cookie is completely incinerated, the amount of heat energy released would be enough to raise the temperature of 1 liter of water by approximately 572°F (300°C)! However, as this system adheres to the laws of thermodynamics, the conversion is not totally efficient (only about 25 percent of energy actually performs cellular work).