When animals or plants are rapidly buried in layers of mud during a natural catastrophe, their bodies are protected from normal processes of decay: scavengers, bacteria, and chemicals are prevented from breaking down their bodies at the usual rate. The hard parts of the animals (such as their bones, teeth, and shells) are eventually replaced with minerals from the mud, which turn them into rock. The soft parts of the specimen, such as the scales of a fish or the leaves of a plant, sometimes leave a colored imprint in rock before they eventually decay. Trees or other organic matter that are covered with silica-rich water become petrified--they turn into solid mineral. All of these methods result in what we know as fossils. A paleontologist is a scientist who studies fossils to find out more about extinct species or about any species that are evident in the fossil record.
Fossils are most commonly found in limestone, shale, and sandstone, all relatively soft rock that erodes more easily than most rocks do. As the rock gradually wears away, the fossil layers within it are exposed. One enjoyable way of demonstrating how fossils are made is to make fossil 'casts' or prints out of plaster.