Bill Nye: The Science Guy

Most dead animals and plants break up, get decomposed, and become part of the soil, but some turn into fossils. A fossil forms when a plant or animal dies, and gets buried. If conditions are right, water gets into the fossil bed, and chemical reactions preserve the impressions for thousands or millions of years. There are different types of fossils — imprints of animals, black carbon outlines, hardened bones, or actual animals and plants that have been trapped in ice or hardened tree sap.