Angels and demons
Here is a modification of a question asked on the xkcd logic puzzle forum. I've modified it slightly so there is (I believe) an answer.
Faced with a particularly vexing moral dilemma, two semitransparent miniature versions of you appear on either shoulder, each offering advise on the situation. From your experience with cartoons, you know that one of these characters is an angel, and the other a demon. Angels always tell the truth if they can, but demons sometimes lie and sometimes tell the truth. Of course, you wish to determine which is which, but you only have time to ask one question of one of the two. What should you ask?