While admiring some dinosaur bones in the Museum of Natural History, a tourist asks the guard, "How old are they?"
The guard replies, "They are 73 million, four years, and six months old."
"That's a rather exact number," says the tourist. "How do you know their age so precisely?"
"Well," answers 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."