Appliance of Science: When is a fossil not a fossil?

Dr Naomi Lavelle

A polished section of fossil wood from the cabinet of Reverend John Henslow, Charles Darwin's mentor at Cambridge, comprising a 150 million years old tree.

We probably all have an idea of what a fossil is, something to do with bones and bits of animals, and plants, that lived a very long time ago. But do you know how old it needs to be to be called a fossil, and what makes a fossil a fossil?

What exactly is a fossil?