There is a prisoner who is about to be executed. The king decides to give him one last chance to live. There are 2 doors, the life door and the death door. There is one guard standing by each door. Those two guards know which door is the life door and which is the death door. However, one of them always tells the truth and the other always tells a lie. There is no way you can identify which door is the life door or the death door. There is no way you can distinguish who is the one telling the truth. The prisoner can only ask one guard one question. Then he needs to choose a door to walk in. If he walks in the death door, then he will be executed. If he walks in the life door, he can have a new life. He did choose the life door and lived. What was the question he asked? How did he choose the door after he got the answer from one of the guards? Reference : http://www.freepuzzles.com/puzzles/PuzzlePage.asp?PuzzleNumber=Logic003&CategoryID=2

You can solve it by this way also. Which door would you point to if I asked you the question which door is the Life door? Both should point to the Life door! If you think of T (true) and F (false) as functions on the parameter Q (question), the T person would reply T(T(Q))=T(Q) and the F person would reply: F(F(Q))=T(Q). For the previous soln, one replies T(F(Q))=F(Q) and the other F(T(Q))=F(Q).

Not always. When I ask the question to the guy who is lier he will point to the death door for the above question.

Yes!!! You are right shabbir Assuming that both know who tells the truth and who lies, ask anyone: Which door will the other person show me if I ask him to show me the life door? And then choose the other door from the answer to the above question. Suppose L and D are doors. And T and F are people. Ans from T: D (as T knows F will answer D (false) and T gives this answer truly) Ans from F: D (as F knows T till answer L (true) and F gives the opposite answer falsely) Since this gives D, we go for the other door L.