A mold fossil is a type of fossil that forms when an organism dies and leaves an imprint in the sediment beneath it. The mineralized impression of the organism left in the sediment is called a mold. If the imprint fills in with minerals from sediment and groundwater, it can harden to form a fossil, which is called a cast fossil. Cast fossils preserve a three-dimensional impression of remains buried in sediment, and the mineralized sediment that fills the mold recreates the shape of the remains. To demonstrate the process of fossilization, one can make mold and cast fossils using materials such as reusable or modeling clay, salt dough, and natural items like seashells or leaves.