There are of course many different types of fossils, but most form through some sort of mineralization process. What’s that? In water that has lots of dissolved minerals, after an organism dies, those minerals can slowly deposit in the tiniest of spaces within that organism, even within cell walls. Well-preserved fossils require the organism to be covered with sediment quickly after death (like on the bottom of a lake) so that the body doesn’t decay before the slower mineralization process can take place.