A man is being held captive in a dungeon. The dungeon has two doors. There ia a guard in front of each door. His captor enters the dungeon and announces to his prisoner that one of the doors being guarded leads to freedom and the other leads to death. He tells the prisoner that he may leave at any time, through either of the doors. Of course, if he chooses the wrong door, he will die. The captor further announces that he may ask only one guard just one question. The prisoner's captor further tells the prisoner that one guard is a perpetual liar and one guard always tells the truth.

What is the one question that the prisoner may ask to one guard, so that he may go free?

Why will that one question work?