Explain the following joke:

A group of school children is on an outing to a museum. Their guide shows them a beautifully restored fossil skeleton of a Tyrannosaurus Rex. "And now, children", she asks, "Do any of you know when the dinosaurs died out?". The little NAME_1 raises his finger. "I know it!", he shouts, "I've been here before. They died out 66 million and two years ago." The response should be in all capital letters.
user