Gasoline is a fuel used mainly in vehicle engines and is made from crude oil and other petroleum liquids. Gasoline is a mixture of many different hydrogen- and carbon-containing chemicals called hydrocarbons, which are organic chemical compounds made up of hydrogen and carbon molecules. The composition of gasoline varies widely depending on the crude oils used, the refinery processes available, the overall balance of product demand, and the product specifications. The bulk of gasoline consists of a homogeneous mixture of small, relatively lightweight hydrocarbons with between 4 and 12 carbon atoms per molecule (commonly referred to as C4–C12) . Gasoline also contains chemicals such as lubricants, anti-rust agents, and anti-icing agents that are added to improve car performance. The exact chemical composition of gasoline is undefined, and the performance specification also varies with season, requiring more volatile blends during winter, in order to be able to start a cold engine. Most of the finished motor gasoline now sold in the United States contains about 10% fuel ethanol by volume, which is added to gasoline mainly to meet the requirements of the Renewable Fuel Standard, which is intended to reduce greenhouse gas emissions and the amount of oil that the United States imports from other countries.