Bob enters a hotel. “How many rooms are there here?” He asks the receptionist. “Twenty,” she replies. “We only have one room left – a single.”
“That will do me perfectly,” says Bob.
“Here you go,” she says. You’re in room number twenty-one.”
Bob looked at the key and sure enough it said 21 in bold numbers on the key ring. Yet the receptionist said there were only twenty rooms. Bob found his room easily and realized how silly he had been.
Why was his room not number 20?