Fossils are the traces left by the remains or activities of ancient creatures. Dinosaur bones or footprints left in the rock strata are all fossils, though the bones are referred to as body fossils while the footprints they leave are referred to as "cast fossils".

When a creature dies, its body usually decomposes, breaks down and eventually disappears. In very rare circumstances, the remains or traces are buried deep under sediment. They are then preserved until everything is replaced by minerals and over time, fossils are created. When the organic matter is replaced by minerals the resulting fossil usually becomes very hard and heavy. The color changes after substitution as well. For example, the silicified wood within the Petrified Forest National Park in Arizona , USA , has been replaced by agate to form kaleidoscope-like colors. The original tissue has long since disappeared but the outline of the growth rings and tree barks remains plain to see.