fossil fuels / fossils forming occurs over a period of 300000000 years which is considered a very long time to a human & fossil fuels / fossils are formed from dead organisms over time ->  fossil fuels are formed from dead organisms over a very long period of time  fossil fuels are formed from dead organisms over a very long period of time & if something formed from something else over a very long period of time then that something else existed a very long time ago ->  fossil fuels are formed from dead organisms from a very long time ago  fossil fuels are formed from dead organisms from a very long time ago & prehistoric means a very long time ago ->  fossil fuels are formed from dead prehistoric organisms  if a living thing dies then that living thing is dead & a living thing is an organism ->  if an organism dies then that organism is dead  if an organism dies then that organism is dead & dead organisms decay ->  if an organism dies then that organism is dead and decays  fossil fuels are formed from dead prehistoric organisms & if an organism dies then that organism is dead and decays ->  fossil fuels are formed from prehistoric organisms dying and decaying  fossil fuels are formed from prehistoric organisms dying and decaying & an animal is a kind of organism ->  fossil fuels are formed from prehistoric animals dying and decaying  fossil fuels are formed from prehistoric animals dying and decaying & oil is a kind of fossil fuel -> oil is formed from prehistoric animals dying and decaying