A fossil is the prehistoric remains of a plant or animal. Fossils are typically preserved when they are buried under many layers of sand and mud. Under great pressure the sand and mud become sedimentary rock. Minerals seep into the fossil replacing the organic matter and creating a replica in stone. A fossil can be a trace fossil or a body fossil.
Trace fossils are signs of plant and animal activity that have been preserved in rock.
For example, dinosaur tracks, trails and dung are all trace fossils. A body fossil is usually composed of the bones of an animal. Most body fossils are made when an animal dies and falls, or is washed into a stream, and is covered with mud. Sometime a fossil is the result of a volcanic eruption. Ash and mud from the volcano can cover animals and plants killing and preserving them at the same time.