Fossils are the remains of ancient plants and animals that have been preserved in rock over millions of years. Fossils form when organisms are quickly buried after death, before their bodies can fully decompose. Over long periods of time, the bones or other preserved body parts become rock. Fossil fuels like coal, oil, and natural gas are formed from the remains of ancient plants and animals. These non-renewable fossil fuels take millions of years to form and provide a significant source of energy when burned, but are being depleted as we use them.