A hydrocarbon is a general term for any organic compound that contains only carbon and hydrogen. Like organic compounds in general, which are derived formally from hydrocarbons by adding functional groups, they can be divided into aliphatic, alicyclic, and aromatic hydrocarbons. Aliphatic hydrocarbons are further subdivided into alkanes, alkenes, and alkynes. Some hydrocarbons, especially terpenes, occur in plant oils, and solid, high-molecular-weight hydrocarbons occur in bitumen, but by far the largest sources of all sorts of hydrocarbons are petroleum, natural gas, and coal gas. They are used as fuels, for lubrication, and as starting materials for a wide variety of industrial syntheses.