What is a fossil?
Fossils are the remains or evidence of ancient living organisms.
Remains include shells, bones and wood. Evidence include mostly traces, such as footprints.
Fossils form when parts of organisms, such as a tooth, are buried by sediment. Over thousands or millions of years, minerals from the rocks replace the original composition of the tooth and form a fossil. Fossils can also form by impression. This is very common in plants, specially in leaves. The shape of the leaf shape and its features remain in the rock as a trace.