Oxygen is very reactive and oxides of most elements are known. A chemical reaction in which an oxide is formed is known as oxidation. The rate at which oxidation occurs varies with the element with which oxygen is reacting. Rust, or iron oxide, for example forms relatively slowly, over days or weeks. Burning or combustion, however, involves a very rapid oxidation. Carbon in fossil fuels, for example, can be quickly oxidised to carbon monoxide and carbon dioxide, with a considerable amount of heat being given off. We can convert this heat into useful energy for heating, electricity and locomotion.