Short joke: Age of dinosaur bones

ome tourists in the Chicago Museum of Natural History are marveling at the
dinosaur bones. One of them asks the guard, "Can you tell me how old the
dinosaur bones are?"

The guard replies, "They are 3 million, four years, and six months old."

"That's an awfully exact number," says the tourist. "How do you know their
age so precisely?"

The guard answers, "Well, the dinosaur bones were three million years old
when I started working here, and that was four and a half years ago."