In physics, energy is a measure of work or potential that, in accordance with the law of conservation of energy, cannot be destroyed or created, only transferred or converted. By work, we mean when a force causes any form of movement or state change, such as heating, melting, evaporating, or changing its shape. For example, when we boil a kettle, electrical energy causes the kettle's element to heat up, converting electrical energy to thermal energy. This energy is then transferred into the water, making it hot and causing it to boil.

The human body requires energy in order to function. We get our energy from food in the form of chemical energy in the food we eat, which is primarily stored in carbohydrates, fats, proteins, and ethanol. The energy content of food is measured in kilojoules (kJ) or, more commonly, calories. The more calories there are in a meal, the more...