A fossil is formed when a prehistoric animal or plant dies and its remains become trapped in soft rock. This often happened in the mud and sand beneath the ancient seas.
The rocks form around the remains of the animal or plant which is then preserved inside the rock. The animal or plant also leaves its imprint in the rock.
Strictly speaking a fossil has to be at least 10,000 years old. Anything less than 10,000 years old is considered by paleontologists (the cool scientists who study these things) to be young.
So, let's find out all about old fossils!