How do fossils form inside rock? Real fossils take millions of years, but you can simulate the process in one afternoon. You press a leaf or shell into wet plaster of Paris and let it harden. When you peel the leaf away, an imprint stays behind. This is how many real fossils formed: an organism left its shape in mud that slowly turned to stone. You can try different objects like shells or small animal models to see how detail changes with shape.