Not sure if it has been posted before, if so please ignore it, and the person who by any chance claims that this "riddle" was already done before will have the next turn.
You're in a dungeon chamber, and there are two doors, one leads to freedom, the other to uh... let's say, death. Each of these doors is kept by a guard. One of the guards speaks only the truth, the other one speaks only lies. You are not sure which one is the liar, and you don't know which door he might keep. The guards know what's behind each door, and they will not keep you from entering any of these doors. Now, you can ask a guard only one question, and you can ask that question to one and only one of the guards.
What is the question?