A noted psychiatrist was a guest at a dinner party, and his hostess broached a subject in which he was most at home.

She said,"Would you mind telling me how you detect a mental deficiency in some person who appears to be completely normal.

He replied, nothing is easier.

"You ask him a simple question which anybody should answer with no trouble. If he hesitates that puts you on the track".

"What sort of a question?"

"Well you might ask him. Captain Cook made three trips around the world, and died during one of them, which one?"

The hostess thought for a moment, then said with a nervous laugh, "You would'nt happen to have another question, as I don't know much of history".