Fossil fuels are a type of fuel that was formed over millions of years from the decomposing remains of plants and animals. These remains were acted upon by microorganisms, fossilized, compressed, and heated underground, until they acquired their current form. Coal, petroleum/oil, and natural gas are all examples of fossil fuels. Whenever you ride a vehicle that runs on gasoline or use electricity generated from coal, you are actually benefiting from the transformative power of decay. (That said, fossil fuels create their own environmental problems and must not be used mindlessly — but that’s a lesson for another day.)