Alternative fuels are derived from resources other than petroleum. Some are produced domestically, reducing countries’ dependence on imported oil, and some are derived from renewable sources. They usually produce less pollution than gasoline or diesel fuel. Ethanol is produced domestically from corn and other crops, producing less greenhouse gas emissions than conventional fuels. Biodiesel is derived from vegetable oils and animal fats. It does less damage to the environment than petroleum-based diesel. Natural gas is a fossil fuel that generates less air pollutants and greenhouse gases. Propane, also called liquefied petroleum gas (LPG), is a domestically abundant fossil fuel that generates less particles that act as physical and chemical air pollutants and greenhouse gases. Hydrogen can be produced from fossil fuels (such as coal), nuclear power, or renewable resources (such as hydropower). Fuel cell vehicles powered by pure hydrogen emit no harmful air pollutants.