Many fossils date from times when there were no people living on earth. Most are so old, and have been buried so long, that they have turned into stone. People have collected and analyzed fossils since ancient times. Footprints, tracks, or other traces left behind by an animal are also called fossils.
Fossils can only form under special conditions.
A fossil usually begins to form when a dead organism is covered up by sand or mud. This happens most often underwater, where eroded silt and clay settle out of the water. Over time, the mud may turn to rock, with the shape of the organism captured inside. Most organisms are destroyed over time. Only a very few turn into fossils.