Fossils can tell us a lot about the past. Think about it: If you were to disappear right now, what clues do you leave behind in your room? What sorts of things can we determine about you based on the clothes you leave behind, or the pictures on your wall?
As it turns out, we can tell a whole lot about the lives of organisms even if they are long extinct. For example, sharp teeth tend to indicate carnivorous diets while broad, flat teeth are meant for chewing hard foods such as plants or nuts. By studying fossils and the rocks we find them in, we can tell a surprising amount about a species.