A gas is one of the four states of matter in which particles move freely and are not bound to a space. The gas form of water is steam. Various physical laws determine the behavior of gas and its response to temperature and pressure including Boyle's law, Charles's law and Avogadro's law. Natural gas is a hydrocarbon mixture which can be used for renewable energy production. Some cars use natural gas as a fuel alternative. Some cars run on 'gas' which is short for gasoline. Exhaust gas which comes from cars is mainly carbon dioxide which is one of the greenhouse gases along with methane which along with other gas emissions have an effect on global warming.