A man stands before two doors and must choose one. One leads to certain death and the other to freedom, but the man does not know which. There is a guard before each door, one of which can only tell the truth and one of which can only lie. Again, the man does not know which. He may ask one question of either, but not both guards and make his choice on that basis. What does he ask?
Old question. The answer is: what would my (brother) or guard say? Once you get the answer from both, which should be the same, do the opposite.