Gas is made of free molecules that move randomly in space at very high speeds. The molecules also collide randomly with each other and if they are heated up they gain more kinetic energy and tend to rise.
2 Additional Answers
Gasoline is a crude oil that is mined from the various oil wells around the world. After mining, the crude oil is separated into either gasoline or petroleum.
Natural gas comes from the Earth through holes in the crust. Gasoline is made of crude oil that has been processed. Most of the gas used in vehicles now has Ethanol added to it. You can find more information here: http://www.api.org/aboutoilgas/factsfuel/