Oil, petroleum, natural gas, gasoline, petrol, coal, coke: all these are types of what people call 'fossil fuels'. So why are they called 'fossil fuels'? Because, like fossils of shells or plants which you can find in some rocks, they are old, often hundreds of millions of years old. In fact, fossil fuels are part of the remains of living things which once flourished on the planet, but died and became buried under thick layers of younger rocks. Coal is the best example of this. If you pick up a lump of coal, it's black and shiny. What made it? Occasionally, you'll find a clue in the form of impressions of plants, usually tree trunks. For coal started out as lush tropical swampy forest, bursting with rapidly growing trees and smaller plants. As they died, more plants grew in the swamps, covering and burying the dead ones whose remains did not decay because they were soaked by stagnant water. No air could get at them. Instead they became peat which got thicker as more swamp forest grew above them. Eventually, the weight of all the material above them became so great it squeezed the peat into the rock you call coal. It is almost pure carbon. And that's where the trouble starts because carbon (coal) will burn in air (oxygen) to make heat. It is this which makes coal and the other fossil fuels so useful for people because the heat from them can be used to make homes comfortable in the winter. It can also be used to boil water and make steam to drive turbines and generators and so produce electricity. And carbon in its liquid form, petroleum, can make all kinds of chemicals and, of course, fuel for transport: cars, trucks, ships and aeroplanes. Petroleum and natural gas are not pure carbon. They are chemicals which contain hydrogen as well. So they're often called 'hydrocarbons'.