Mesozoic names a long stretch of Earth's past, from about 250 million years ago to about 65 million years ago. This is the age of the dinosaurs. Birds appear. Plants with flowers start to grow. The land and the seas slowly become a world we would half-recognise.

Scientists cut Earth's story into big chapters. The Mesozoic is the middle chapter of the time when animal life leaves an easy trail in the rocks. Before it comes the Paleozoic; after it comes the Cenozoic, the chapter we still live in. Inside the Mesozoic sit three smaller ages: Triassic, Jurassic, Cretaceous. Films often say 'Jurassic' for all dinosaurs; though actually Jurassic is only the middle slice.

The chapter ends with a famous dying-out, often linked to a huge rock from space. The giant dinosaurs disappear. Though actually, birds are still here, and many scientists now say birds are dinosaurs — so 'the dinosaurs all died' is a clean sentence that is not quite true.

You will see Mesozoic in science books and museum signs, usually as Mesozoic Era. It is a specialist word, not café talk, but school texts use it. Capital M.