Combustion occurs when fossil fuels, such as natural gas, fuel oil, coal or gasoline, react with oxygen in the air to produce heat. The heat from burning fossil fuels is used for industrial processes, environmental heating or to expand gases in a cylinder and push a piston. Boilers, furnaces and engines are important users of fossil fuels. Fossil fuels are hydrocarbons, meaning they are composed primarily of carbon and hydrogen. When fossil fuels are burned, carbon dioxide (CO2) and water (H2O) are the principle chemical products, formed from the reactants carbon and hydrogen in the fuel and oxygen (O2) in the air. The simplest example of hydrocarbon fuel combustion is the reaction of methane (CH4), the largest component of natural gas, with O2 in the air. When this reaction is balanced, or stoichiometric, each molecule of methane reacts with two molecules of O2 producing one molecule of CO2 and two molecules of H2O. When this occurs, energy is released as heat.