500
"If I had asked you if this was the door to freedom, would your answer have been 'yes'?"
You are stuck in a dungeon with two doors. One leads to freedom, the other to death. You may ask just one single question of the two guards standing by the doors in order to escape. One of the men always lies and the other always tells the truth, but you can't differentiate the liar from the truth-teller.
What one question can you ask that will guarantee your exit through the right door?