Nov 10th 2012, 2:04:24
2 guards stand between two doors. One door leads to paradise, the other to death and suffering. One guard always tells the truth, and the other always lies. You don't know which is which. You can ask one of them, one question. What do you ask to determine which door leads to paradise?
The twist: The guards do not know anything about each other, and so can not tell you how the other would answer.
I just came up with this a few minutes ago while I was reading today's XKCD comic and it is (as far as I know) original. See if you can work it out.
-Fooglmog
Guy with no clue.