## The Honest and Dishonest Guards

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.

What question should you ask?

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.

36 Comments on "The Honest and Dishonest Guards"Aswathy says

April 6, 2015 @ 18:23

Ask them, ” what would the other person say is the right door?” Both would lie, so pick the other door. Simple!

V says

April 24, 2015 @ 19:03

But how would you know which guard told the truth and which one was lying if you could only ask one question to one guard?

Aravind says

July 15, 2015 @ 11:42

This is how:

Go to any of the guards.

Ask him this question :

“If I ask that guard which is the correct door what will he say?”

If it is the liar, he will lie. If it is the truth teller, he will be saying about the other person, which is liar.

So if you ask this question to any one of them you will get a wrong answer.

But there are only two doors, and one of them is wrong.

So after asking that question, go to the door which he is NOT saying about.

Aravind says

July 15, 2015 @ 11:45

Ask any one of the guards this question:

“If i ask the other guard which the right door is what will he say?”

Then he will show us a door. If we asked a liar, he will lie. If we ask truth teller, he will lie because he is talking about the other guard, who is a liar.

So go to the other door than which he points to.

Haley Dewings says

October 13, 2015 @ 22:21

Easy! Ask, ‘Do both of the doors lead to the exit?’

The liar would say yes, and it’s pretty obvious which door to go through after that. I know I got it right!

rjsavvy says

October 15, 2015 @ 03:10

Just ask “what colour are my eyes” the liar will lie and you’d know or be told the truth and you’d also know!! Simple even if it isn’t the “correct” answer ?

Seth says

November 16, 2015 @ 12:52

Some of the commenters arent getting the point; if I asked one of the gaurds what color my hat or eyes or anything was, id find out which gaurd lies and which tells the truth. BUT…. in doing so, you waste your ONE question.

For those having trouble understanding the answer, follow this logic:

When asked, “which door would the other guy tell me is correct” you will get the wrong answer from both gaurds, which leaves only the correct door.

How? If you ask the liar, he will lie, giving you the wrong answer. If you ask the truth teller, he will give you the liar’s answer. So they both give you the wrong door. Which leaves only the correct door

Tony says

December 22, 2015 @ 14:34

How? If you ask the liar, he will lie, giving you the wrong answer. If you ask the truth teller, he will give you the liar’s answer. So they both give you the wrong door. Which leaves only the correct door

The liar will lie telling the truth and the honest guard will lie because the is telling the truth about the liar which is the opposite so you are back where you started.

Dan says

December 22, 2015 @ 15:16

@Tony Did you read the explanation in the answer?

Bonnie says

February 13, 2016 @ 15:30

Here’s another, albeit more confusing, version of the answer:

Ask either guard, “What would you say if I asked you which is the correct door?”

If you have asked the truth teller, he will be completely honest. If you had just flat-out asked him which door is correct, he would have been honest, so he will tell you the correct door.

If you had flat out asked the liar which door is correct, of course he would have lied. However, he must lie about the fact that he would have lied when faced with the question above. As a result, he will tell you the correct door.

No matter which guard you ask, therefore, you will be told the correct door.

bob says

April 5, 2016 @ 20:50

ask either guard: is the guard to paradise a truth teller.

if hell’s guard is truth, he will say no since heaven’s guard needs to lie

if hell’s guard is lie, he will say no since heaven’s guard will tell the truth

if heaven’s guard is truth, he will say yes since heaven’s guard will tell the truth

if heaven’s guard lies, he will say yes since he lies that he tells the truth

Jake says

April 11, 2016 @ 23:39

You ask either guard what the other guard would tell you is the door leading to life and the door leading to death and pick the opposite door of which they said.

If you ask that to the truth teller, he will say the lie that the lie teller would say to you which will give you the door leading to death and then pick the opposite.

If you ask that to the lie teller he will say the opposite of what the truth teller would say which would give you the door leading to death and then pick the opposite.

Sabbatai says

May 3, 2016 @ 21:50

Rjsaavy, you’ve now used your one question. You cannot ask either guard any questions despite knowing which is the liar and which tells the truth. Now you have to choose the door on your own.

Haley Dewings… what? How would that help you. Lol… it’s “obvious which door to go through” based on what? You’d be taking a 50/50 chance and your question to the guards would not have helped at all.

Yeshua Alexander says

May 22, 2016 @ 04:35

I have a solution I haven’t seen posted on this forum or any other: ask the guard if a liar stands in front of the door to death. If he says yes take his door. If he says no take the other.

I could explain but its tedious and i think simulating all 4 possibilities for yourselves would be more reassuring.

Eddie says

May 28, 2016 @ 16:48

C potato

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

onequestion toeitherguard, 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

Shower scene | Paul Hassing's autobiography. says

January 6, 2017 @ 17:23

[…] She seems genuine, but I’m reminded of a childhood riddle. […]

Bryant says

May 2, 2017 @ 23:02

The answer is as simple as math. I would ask either guy to put the correct answer to (for example) 9×9 on the door that leads to freedom. If its the guy that always lies he would put the wrong answer on the wrong door because he always lies. If it’s the guy that tells the truth he would put the correct answer on the correct door.

Rookie says

May 14, 2018 @ 10:10

I’d ask the first guard something I know the answer too – ‘how many fingers am I holding up?’ ‘What is the colour of that door?’ etc. From that answer, I can figure out if he tells the truth or lies, and figure out the other one.

Then I ask the other one which door is the correct one, and choose the one depending on if they are a liar or not.

Outcome 1) The first guard lies, so I pick the door the other guard tells me

Outcome 2) The first guard tells the truth, so I pick the door that the other guard doesn’t tell me.

Missing The Point says

June 7, 2018 @ 15:12

I would just go out from the the door I came in. :-)

Zachary Hemmer. says

June 11, 2018 @ 22:27

I thought about this one for a while, and I finally found an answer.

just ask “If you were the other guard, what would you say if i asked you what the other guard tells?”

the lair will say truth.

the honest will say lair.

Lying guard logic and answer.

Knowing that the other guard is honest, he knows that the other guard would be honest and say that he was the lair, so he lies and says “I would say ‘he tells the truth’”

Honest guard logic and answer.

Knowing that the other guard lies, he knows that the other guard would say that he was the liar, because he always lies. So he answers truthfully, saying “I would say “he is the one who lies”

Problem Solved

Chuck Cochems says

June 13, 2018 @ 00:57

The puzzle, as stated, is invalid. It cannot be solved.

Here’s why.

The lying guard can lie by saying “i don’t know”. This is definitely a lie, since the lying guard does know which door is correct. In fact ANY “If I were to ask you” question directed at the lying guard can be answered falsely with “I don’t know”. We know it’s a lie as soon as we hear it. But we gain no information other then the fact we asked the liar.

if we ask one guard about the other guards answer, we don’t get our answer either way. The liar can answer ” i don’t know” which is a lie, because he does know what the truth teller will say. The truthteller must either answer “I don’t know” (because the liar has two options) or “he will say he doesn’t know” (making the reasonable assumption that the liar wants to get you killed)”. either way you are screwed and do not find out which door is correct.

Asking the truth teller about his own answer does reveal the correct door, but you only have a 50/50 shot of getting the truth teller. it’s no different from just asking the truth teller directly.

If the problem is stated this way it is solvable.

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 can only tell lies. Neither guard desires your death. 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.

What question should you ask?

That qualifier about the lying guard not actively trying to get you killed means that he won’t just say “i don’t know’ which is what a malicious lying guard would do, which makes the puzzle solvable.

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