Some tourists in the Punjab Museum of Natural History were marveling at the dinosaur bones. One of them asked the guard, Santa Singh ''Can you tell me how old the dinosaur bones are?'' Santa replied, ''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?'' Santa answered, ''Well, the dinosaur bones were three million years old when I started working here, and that was four and a half years ago.''