Fuels are all substances that give off heat and light energy when burned. Thermal energy is obtained from various fuels by a chemical process called oxidation or combustion. In most cases, oxidation is the coupling of combustible matter with oxygen. Combustible substances are otherwise organic substances, i.e. hydrocarbons. Thus they contain a large proportion of carbon and hydrogen. Consequently, burning produces a large amount of carbon dioxide and water vapour and depending on the composition of the fuel and other compounds.