## 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.

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.

Posted in Brain Teasers

55 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!

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.

“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

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.

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.

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

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

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…..

@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. :)

December 29, 2016 @ 14:22

or you can ask the guards what door they go out of after shift

[…] 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

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.
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’”
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.

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.

Ezzy says
July 28, 2018 @ 03:52

I would ask either one; ‘if you are a liar, tell me me the exit door?’

Being an honest guard, he will still be pointing the exit door. The liar will lie to his lie (double negation) thus points to the exit door.

Captain Dave says
August 9, 2018 @ 04:25

Easy…. Kill one guard and ask the remaining one, “Is he alive?” *pointing at the dead one*
That guard will answer Yes/No depending on which he is (Liar or Truth Teller), then the follow up based on his answer would be, “Which Door is safe?”

Only one question Captain Dave!

Dave says
August 31, 2018 @ 12:07

Let’s say that the two doors lead to prison and freedom for this scenario.

The way to solve this is to simply ask one guard, “What will the other guard tell me if I ask him where his door leads to?”

For this scenario, let’s say that freedom is behind door #1

Ask the guard in front of door #2, “What will the guard in front of door #1 tell me if I ask him where his door leads to?”

If he is the truthful guard and the other is the liar, then he will say, “He is going to tell you prison.” Because the guard in front of door #1 lies, which would mean that the door #1 really leads to freedom, not prison.

If the guard in front of door #2 is the liar he is going to say, “He is going to say prison.” Because guard #1 tells the truth which means door #1 leads to freedom. And the guard at door #2 lied to you about what the other guard’s answer would be.

So the only way to solve the puzzle is to ask one guard what the other would say and you know that the door would be just the opposite of the answer.

November 24, 2018 @ 06:46

I would push one of the guards through the door then shout “are you ok?” If I get no answer then it was the death door :)

Bren says
January 9, 2019 @ 14:11

It says error the guard not both

Mike D says
April 23, 2019 @ 01:38

Ask either gaurd what would the other gaurd tell me is the door to life, then go through the oposite door.

Billy says
July 14, 2019 @ 16:15

This riddle is unsolvable. Because you are only allow to ask one question to the guards once relating to the doors. Not one question each and has nothing to do with the door. So the moment ask the guards any question, they will proceed to answer you at the same time. The liar cant simply lie and say the same thing the truthful one is saying. This will pretty make chance back square 1. You wont know.

Ali says
July 20, 2019 @ 07:39

Ask either gaurd what would the other gaurd tell me is the door to life, then go through the oposite door.

proof is ,
minus into pluse minus

suppose
lier is minus( negative)
and true is plus (positive)
-*+ is equal to -(minus)

practically approved

thakur raina says
August 16, 2019 @ 13:50

the problem starts YOU ARE IN A ROOM WITH TWO DOORS And the prisoner knows from which door he was brought inside the room. He canchoose that door for exit. Why he should ask any guard.

Two Questions says
August 20, 2019 @ 10:14

If I can ask two questions at both Guards then there they are:

Question 1: Can the Guard who Speaks the Truth Lie it’s Truth?
-The Honest Guard will say no while the other will say yes.
Question 2: Which door is the Exit?
-By knowing which one lies and such then you already know who’ll point to the Exit.

Quak says
September 20, 2019 @ 17:21

If You ask A guard If the other one is alive, If he says no, You have the liar. Ask him which door is safe, When he tells you, Go through the opposite side.

Mark says
November 29, 2019 @ 15:01

To ask the right (single) question you must focus on the answer, not who is lying or telling the truth. Once you get the answer you know which door leads to freedom and you go through it without knowing which guard is the liar. You don’t dare,

Ask either guard (or both at the same time) “Is the honest guard standing by the door to freedom?” Whoever says “yes” is standing by the freedom door. The other guard will say “no” or “I don’t know”, anything but “yes” so don’t go through that door.

Honest-Freedom door…Yes
Honest-Prison Door……No
Liar- Freedom door…Yes or something else, but not no
Liar- Prison Door…….No or something else, but not yes

Remember you don’t care who is lying when you phrase the single question this way. But if the liar obfuscates instead of giving a yes or no you can assume he is also the liar. If the liar says yes or no then you don’t know who the liar is but you don’t need to know.

RoRoRowyerBoat says
December 7, 2019 @ 18:01

Ask a question that you already know the true answer to: Is the Earth round? Does 1 + 1 = 2? Is my name (whatever your name is). The dishonest guard can only lie, so he will always give the incorrect answer to these type of questions. Et voila.

Dave Griffiths says
January 9, 2020 @ 11:07

I would look at one guard’s shoes. If they were black ( or any other color ) I would ask if their shoes were black. If he says yes he is the truthful one. If he says no then he is the liar. I would then enter the truthful ones door.

chlocartoon says
April 7, 2020 @ 01:40

“at this exact moment, do I have hands?”

I do. If they say no then they are the liar and I will choose the other door, etc.

or I would ask them an obvious math question, like
“what does 2+2 equal?”

Vivas says
April 11, 2020 @ 10:30

No, I won’t have to ask any question from any of the two guards. I’ll just await which door any or both of them will use when their guarding shift is over.

Ryan Veasman says
September 12, 2020 @ 02:24

Hi.
I would like to post this, though it has been posted already.
Just because you know who tells the truth does not mean that his door is the right one!!!!!!!!! Can you hear me? Magical truth man does not always stand in front of the right door! You only know who tells the truth. And youbwasted your question to know it. So great, you found out that the truth teller can do math and 2+2=4. Whoopdee-doo. That doesn’t mean that the truth tellers door is the right one. You’re missing the point. You’re not trying to figure out who lies and who tells the truth. You’re trying to find the exit. PLEASE stop posting just ask something obvious and go through the truth teller’s door. One question to one person. Then you go through one door. Correct answers explained above among a jillion wrong ones.

Tony says
October 23, 2022 @ 16:28

Ask the other soldier which is the right door and say tell me what he says, then the true soldier will tell you the wrong door that the false soldier told you, but the false soldier will tell you the right door that the true soldier told you wrong, the two doors are one, then you can escape through the other door.