Fossils can be sorted, or classified into different types depending on how they formed.
Many fossils are of shells, bones or teeth that have been turned to stone by a process called pertrification.
There are other types of fossils too. Some fossilized animals were not turned to stone but simply preserved when they became trapped in amber, tar, peat or ice. Stony fossil may also preserve a mold or outline of living thing, rather than preserving the organism itself. Some times the traces left by an animal or plant during its life such as footprints in mud can be fossilized.