Fossil fuels include oil, coal and natural gas. Fossil fuels come from animals and plants that have died millions of years ago and decomposed to make a useable source of energy for us humans. The problem is that we are now running out of Fossil fuels and they are becoming more expensive, this is because we keep digging it up from the ground and we expect there to still be more but you can only have so many dead animals and plants underground to create the fossil fuels. Crude oil, that is a fossil fuel, is dug up from the ground and then put in a machine to heat it up. As the crude oil chains break up between them they move to another machine that cools them down. As the heated crude oil chains break some chains will be smaller than others, which means they will have different cooling points. When the crude oil is heated up it then moves to another machine that cools the crude oil down. As the different chains have different boiling points the evaporated crude oil will travel towards the top, and as it is cooled it could turn into a liquid. But if the chain is smaller it will just continue towards the top and come out of the machine as a gas. However, the big chains will come out as a sludge type of substance. This is called a distillation process.