While admiring some dinosaur bones in the Natural History Museum, a tourist asks the guard:
- How old are they?
The guard replies:
They have 73 million, four years and six months.
- It's a pretty accurate number, says the tourist. "How do you know their ages so precisely?"
- Well, replies the guard, the dinosaur bones were seventy-three million years old when I started working here, and that was four and a half years ago.