Ethylene is a hydrocarbon with the formula C2H4 or H2C=CH2. It is a colorless flammable gas with a faint “sweet and musky” odor when pure. It is the simplest alkene, and the simplest unsaturated hydrocarbon after acetylene (C2H2). Ethylene is widely used in chemical industry, and its worldwide production (over 109 million tonnes in 2006) exceeds that of any other organic compound. Ethylene is also an important natural plant hormone, used in agriculture to force the ripening of fruits.