Slightly off-topic, but have you ever seen a movie called The Strange Enigma Of Kaspar Hauser? One of those sort-true stories; Kaspar was kept in a shed by his parents until he was nearly thirty, and when he finally emerges into the world he's quite a sensation.
One of the local wise men tries to teach him logic. He uses the old riddle about meeting two men at a fork in the road. You need to ask these men for directions, and you only know that one of the men comes from a village where they always tell the truth, and the other from a village where they always lie. You need to ask a question which will tell you which you can trust. I forget the correct question and the logic behind it, but Kaspar listens to it all and then says, 'I would ask him a different question.' The wise man says, there is no other question. That's the answer to the riddle. Kaspar insists, 'I would ask him a different question!' The old guy gets all huffy and says, 'All right then, young man, what question would you ask?'
Kaspar replies, 'I would ask him if he is a frog.'