Say to the first guy.."if I asked the other guy which door to choose, what would he tell me?"\n\nThen choose the "other door".\n\nIf you asked the truthful one, he will tell you that the lyer would sugest the wrong door.\n\nAnd if you asked the lyer, he would know which door the truthful would say, but, being a lyer he will give you the wrong answer.\n\nEither way, if you pick the other door you win!