You’re in a room with two doors. There’s a guard at each door. One door is the exit, but behind the other door is something that will kill you. You’re told that one guard always tells the truth and the other guard always lies. You don’t know which guard is which. You are allowed to ask one question to either of the guards to determine which door is the exit.
Ask either guard what door the other guard would say is the exit, then choose the opposite door.
If you ask the guard who always tells the truth, he knows the other guard would lie, so he’ll point you to the door leading to death. If you ask the guard who always lies, he knows the other guard would truthfully show you the exit, so he’ll lie and point you to the door leading to death.
An alternate solution is to ask a guard what they would answer if you were to ask them which door was the exit, then choose that door. The truthful guard will point to the correct exit, but the lying guard will too. Here’s why. If you asked him what door was the exit, he would normally lie and point to the death door, but you asked him what he would say if you asked what door was the exit, and in order to lie to that question, he will point you to the exit.
If you asked the lying guard which door the other guard would say, the lying guard would say the wrong door so you would go to the opposite. If you asked the truthful guard what the lying guard would say they would tell you truthfully which one is the door the lying guard would say and you would go to the opposite of what the lying guard would say.
After asking either of the guards what the other guard would say is the right door, go to the opposite of what they say. If you ask the lying guard what the truthful guard would say, they would tell you the opposite of what the truthful guard would say so you would go to the opposite door. If you ask the truthful guard, they will tell you what the dishonest guard would say and you would go to the opposite door of what the guard says.
An outside-the-box solution is to ask a simple question (“What does one plus one equal to?”) to find out who is the liar. You can then punch whoever turns out to be the liar through the door, and observe what happen.
“Is there an exit behind one of these doors?” You already know there is an exit behind one of the doors. If you ask the truth teller he will say yes and you know that is the door you should pick and if you ask the liar he will tell you there is no exit behind either of the doors so you will choose the other.
ask one gard what colour is my hair if he lies ask the truth teller what the right door if the truth teller answers ask the liar which one is correct choose the opposite door then you live!
Well this is a good one: Three watertight answers:
1. Ask the guard which door is the door to freedom, take the one they do not recommend
2. Ask the guard, if I asked you yesterday which was the door to freedom, take the door the gard recommends
3. Buy me dinner and I may tell you the third
Excellent riddle. With in the comments, there seems to be confusion though. Firstly, the guards are not standing in front of each door, they are separated… as one commenter seemed to express with the comment, ” What’s 2+2?”. 2 guards, two doors… only ONE question to either guard. Keeping this in mind limits the thought train… sometimes, in theoretical circumstances, its easy to get off track. With the abilitie to ask two questions, the possibilities are nearly endless with cleverness. But, back to the one question, there really is only one choice in this matter. You must ask about the two doors… Asking directly about what choice each would give as correct, will only confuse. BUT, asking another theoretical question to either guard would reveal the bad choice… that question is, ” What would the OTHER guard say is the exit door?” At this, both would choose the death door. Because, sadly, the truth can speak a lie for the sake of truth, but a liar just, lies. Only each character knows who they really are. :)
55 Comments on "The Honest and Dishonest Guards"
No Name says
June 10, 2016 @ 17:39
If you asked the lying guard which door the other guard would say, the lying guard would say the wrong door so you would go to the opposite. If you asked the truthful guard what the lying guard would say they would tell you truthfully which one is the door the lying guard would say and you would go to the opposite of what the lying guard would say.
No Name says
June 10, 2016 @ 17:44
After asking either of the guards what the other guard would say is the right door, go to the opposite of what they say. If you ask the lying guard what the truthful guard would say, they would tell you the opposite of what the truthful guard would say so you would go to the opposite door. If you ask the truthful guard, they will tell you what the dishonest guard would say and you would go to the opposite door of what the guard says.
Sam says
June 13, 2016 @ 20:29
Whats 2+2?
Zac Tolan says
June 18, 2016 @ 04:13
I learned this is 6th grade math. The first question you ask is, “Can I ask you a question?”
Mystery Kid says
July 10, 2016 @ 20:28
Ask Him “Are you going to answer yes to this question.”
ecstasy says
July 27, 2016 @ 17:16
just because he is a liar it doesn’t mean he is not standing infront of the exit door!
Aristide T. says
August 10, 2016 @ 15:34
An outside-the-box solution is to ask a simple question (“What does one plus one equal to?”) to find out who is the liar. You can then punch whoever turns out to be the liar through the door, and observe what happen.
Lilly says
August 24, 2016 @ 16:15
“Is there an exit behind one of these doors?” You already know there is an exit behind one of the doors. If you ask the truth teller he will say yes and you know that is the door you should pick and if you ask the liar he will tell you there is no exit behind either of the doors so you will choose the other.
sreevidhya says
October 18, 2016 @ 05:02
beat one of the guards. If he is the liar .he will say he didn’t beat me…
That’s all… Ask the question? And escape.
bob says
October 24, 2016 @ 22:44
ask one gard what colour is my hair if he lies ask the truth teller what the right door if the truth teller answers ask the liar which one is correct choose the opposite door then you live!
Alex says
October 30, 2016 @ 22:24
Actually you are allowed to ask EACH guard ONE question…….So you can ask one guard what your eye color is….and ask the other which door is the exit…..
Dan says
November 1, 2016 @ 08:20
@Alex Actually, you are only allowed to ask one question to either guard, not both.
Gavin winship says
November 26, 2016 @ 20:13
Well this is a good one: Three watertight answers:
1. Ask the guard which door is the door to freedom, take the one they do not recommend
2. Ask the guard, if I asked you yesterday which was the door to freedom, take the door the gard recommends
3. Buy me dinner and I may tell you the third
Anette says
December 6, 2016 @ 06:58
Excellent riddle. With in the comments, there seems to be confusion though. Firstly, the guards are not standing in front of each door, they are separated… as one commenter seemed to express with the comment, ” What’s 2+2?”. 2 guards, two doors… only ONE question to either guard. Keeping this in mind limits the thought train… sometimes, in theoretical circumstances, its easy to get off track. With the abilitie to ask two questions, the possibilities are nearly endless with cleverness. But, back to the one question, there really is only one choice in this matter. You must ask about the two doors… Asking directly about what choice each would give as correct, will only confuse. BUT, asking another theoretical question to either guard would reveal the bad choice… that question is, ” What would the OTHER guard say is the exit door?” At this, both would choose the death door. Because, sadly, the truth can speak a lie for the sake of truth, but a liar just, lies. Only each character knows who they really are. :)
Ian Adkins says
December 29, 2016 @ 14:22
or you can ask the guards what door they go out of after shift
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