When commonly sold as fuel, it is also known as liquifed petroleum gas (LPG or LP gas) and is a mixture of propane with smaller amounts of propylene, butane and butylene. It is used as fuel in cooking on many barbeques and portable stoves and in motor vehicles. Propane powers some buses, forklifts, and taxies and is used for heat and cooking in recreational vehicles and campers. In many rural areas of the US, propane is also used in furnaces, water heaters, laundry driers, and other heat-producing appliances. Delivery trucks fill up large tanks that are permanently installed on the property (sometimes called pigs) or exchange bottles of propane.