Fuel, substance that reacts chemically with another to produce heat, or that produces heat by nuclear processes. The term fuel is generally limited to those substances that burn readily in air or oxygen, emitting large quantities of heat. Fuels are used for heating, for the production of steam for heating and power purposes, for powering internal-combustion engines, and for a direct source of power in jet and rocket propulsion. In cases where a fuel must supply its own oxygen, as in many rockets and torpedoes, an oxidizing agent such as hydrogen peroxide or nitric acid is added to the fuel mixture. Fuels usually come from alcohols that are very flammable and burning them result in large amounts of energy, thermal energy, that energize the engines that use it. Usually, fuels are derived from crude oil derived from fossils where organic matter that is buried several hundreds of meter below the earth and fermented for hundreds of years. The biological derivatives are rich sources of octane that is necessary for combustion. Octane ratings can be observed to be indicated in gasoline stations, where a higher octane rating is indicative of a greater fuel efficiency.