Fossils are naturally-preserved physical traces of long-dead organisms. Usually, these traces consist of an organism's hard parts, such as bones, teeth, shells, or wood. Occasionally, when conditions are optimal, soft parts of organisms can also fossilize, such as impressions of skin, body outlines, and, more commonly, leaves. Other traces of objects made by organisms, such as footprints, burrows, and nests, also qualify to be called fossils. Most definitions of fossils require that the organism's body part or other physical trace be more than 10,000 years old in order to truly be called a fossil.