Some 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."
The tourist replies, "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."