A cast fossil is a type of fossil that forms when an organism dies, and its flesh decays and bones deteriorate due to chemical reactions. Minerals gradually enter into the cavity, resulting in a cast, which is in the general form of the original organism. Casts and molds are types of fossilization where the physical characteristics of organisms are impressed onto rocks, especially coarse porous rocks such as sandstones. Typically, the hard parts of an organism (shells of mollusks, skeletal structures of coelenterates, bones and teeth of vertebrate, chitinous exoskeleton of arthropods, trunks of trees, and many sphenophyte) leave the best impressions. These hard structures are usually composed of calcium carbonate, calcium phosphate, silica, or chitin, and do not decay as easily. The rigidity of the hard body parts also allows the sediment to form around the organism. Soft body parts decay too fast for impressions to form and are not rigid enough for a mold to set around. Cast fossils are formed when an organism leaves a print in the mud, which is subsequently filled with sediment. The filled mold becomes the cast.