A fossil is the preserved traces or remains of any ancient creature. In addition to their bones and teeth, footprints and even droppings left behind by dinosaurs, qualify as fossils. Dr David Norman with a fossilised bone Fossils are one of the main pieces of evidence for the evolution of life on Earth. To become a fossil an organism has first to be buried. If not, it will decay or be scavanged by other creatures, and be totally destroyed.
Most creatures completely disappear after they have died.
Their body chemicals are recycled in the biosphere as part of Nature's economy. So fossils are actually special and quite rare. They represent creatures that have cheated the normal cycle of decay and renewal by becoming buried. These are produced by natural weathering and are generally called sediments. An animal must be covered with sediments before it has completely rotted, in order to become a fossil.