A fossil is proof of a once living organism. Fossils are created when things like bones, shells, teeth, footprints, leaves and claws are left behind, sediment (for more info on sediment go to "sedimentry rock page") covers the remains. Layers of sediment collect, the weight of the sediment causes compaction (for more info about compaction go to "compaction and cementation page"). The remains are preserved in the compacted sediment.