Natural gas contains about 90% methane in pipeline form. Methane is the simplest hydrocarbon, making natural gas the cleanest fuel currently available for vehicles. CNG is compressed to around 3,600 psi (24,820 kPa) and stored in high pressure tanks. LNG is cooled to about -263ºF (-164ºC) in a process called liquefaction to condense it to a more pure liquid form. The liquid form is more dense, containing more energy than CNG, but is more complex to produce and transport. CNG and LNG have found their best fit so far in fleet vehicles, where it is easier to adapt to a reduced driving range and limited access to fast refilling stations. LNG is generally used in heavy-duty fleet vehicles like trains, transit buses, and semi-trucks. CNG is often found in fleet vehicles like cars, vans, trucks, and school buses.