Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Usually low levels of trace gases like carbon dioxide, nitrogen, hydrogen sulfide, and helium are also present. Natural gas is colorless and odorless, so odorizers such as mercaptan, which smells like sulfur or rotten eggs, are commonly added to natural gas supplies for safety so that leaks can be readily detected.