"Would you mind telling me, Doctor," Bob asked, "how you detect a mental deficiency in somebody who appears completely normal?"
"Nothing is easier," he replied. "You ask him a simple question which everyone should answer with no trouble. If he hesitates, that puts you on the track."
"What sort of question?"
"Well, you might ask him, 'Captain Cook made three trips around the world and died during one of them. Which one?'"
Bob thought for a moment, and then said with a nervous laugh, "You wouldn't happen to have another example would you? I must confess I don't know much about history."