Fossils are the geologically altered remains of a once-living organism and/or its behaviour. There are two main types: body fossils represent all or part of the organism’s body, and trace fossils show evidence of the organism’s behaviour. The study of fossils is called paleontology, from the Greek meaning ‘ancient (paleo-) being (onto-) study (-logy)’. Scientists who study fossils are called paleontologists. Fossils are of particular interest to geologists as they can help determine the age of a particular rock compared to other rocks, or identify the environment in which the rock was formed.