Problem of the Two Doors: Classic Logic Puzzle

A classic logic puzzle / riddle. Are you up to the challenge? (With support and, finally, the solution.)

[thomasgrable]

"One of us always tells the truth, the other one always lies."
"Oh my god, Carl, I said I was sorry!"

[nigeria]

"barbarian takes axe and kills first guard" "is he dead?"

[brigittakovacs]

'Takes barbarian axe kills one'
"Is he ded"?
"No"
"This one lie"

[benji]

don't ask a question, just go to one door, open it and push the guard through. If you hear screams of death then that was the danger door.

[kevindomenechaliaga]

For those who didn't understand: Asking what door the other guard would tell me is safe, always leads to a false answer. If we ask the liar guard ¿What would the other guard say, if i asked wich door is safe? The liar guard would point to the damger door, since the other truthful guard would do the oposite. If we make the same question to the truthful guard, it would answer as the liar, and also point at the danger door. Thus, asking ¿What would the other guard say if we asked wich door is safe? always has the danger door as an answer. This way, we know wich is the danger door, and that the remaining door is the safe one :D I hope this explains it more concisely

[AdultToons]

Sometimes I need shit explained to me like I'm a newborn.

[rohankishibe]

This was very helpful. Usually I can never understand this riddle, but this helped me understand it.

[rabbithedragon]

This puzzle becomes next level when there's only 1 door 1 guard and he lies half the time but you get 3 questions to figure out weather it's currently safe to walk through the door.

[wraithscythe]

Our astronomy teacher my sophomore year in high school gave us this problem at the beginning of the semester just for fun. End of the semester I figured it out for the whole class. One of the best teachers I ever had.

[BloodyAlex]

"How many fingers am I holding up?"

[Ozinarg]

This riddle is wrong. You get one question total, not one question per guard. Otherwise, you could just ask gaurd number 1: "whats 2+2?" If they lie, you know the other gaurd is the truth teller, and you can safely ask them which way is safe. If they do not lie, you know the other guard is the liar, and ask them which was is safe and then do the opposite of what they say.

[Pelerin]

"Do you have your helmet on your head right now?"
"No."
"Thank you to be such a good liar."

[zg]

Finally, after all these years, that episode of Yu-Gi-Oh now makes perfect sense to me. Ever since watching that episode I thought it was madness but alas it was shear brilliance and you explained it so simply and elegantly. Well done ✅.

[mingmanangan]

if i ended up in that situation... i'll just go home and won't bother those guards :)

[theblackheart]

I doubt I’ll ever wrap my head around this conundrum. Thanks a lot Ricky, Steve, and Karl.

[LNCP]

Thanks for explaining the riddle from "The Labyrinth"!

[josephrusso]

The key factor that you omitted is that each guard knows that the other guard will respond in the opposite way. There are actually two ways of asking this question, you can ask about the door that leads to death which will reverse your response based on a yes or no answer. So there are two ways of asking this with 8 possible scenarios.

[RandomsFandom]

You dont ask the guards about what is behind them, you ask them about what is behind you.

[dariusq]

Anyone else get "Labyrinth" flashbacks? 😂

[patrickrannou]

Many D&D RPG players know this riddle. So I made the room with two undead knights, and an undead priest. The undead priest is the one presenting the riddle. So the players assume the typical two doors puzzle situation, so ask the question the "good solution" way, asking one knight what the other knight would say. If they try to ask the priest he says he doesn't know which door is the safe door. So they get a door that they "logically" think is the safe one, and go there.

However, their entire premise is wholly faulty: Nothing proves to the players that the undead priest is actually telling the truth about the entire situation! In fact the 3 undeasd can say whatever they want! Both doors simply lead to more dangers, and the "good solution" of assuming the undead priest said the truth, actually leads to the worse of these two rooms. The 3 undead in the room know they can't beat the PCs directly in a fair fight, so they hope to avoid direct combat until the PCs have to start facing the threat of one of the next rooms, and then quickly arrive to attack the PCs "in their back", while they are already busy fighting! Suddenly making the fight way tougher.

The only "hints" that the PCs should not believe the priest, is a previous encounter with the same type of undead knights (they all have the same heraldry symbol on their shields armor robes and vestments), to show how lying, cunning, and manipulative, such undead can be, with that 1st undead trying to befriend the PCs, offering to act as their guide to lead them to the treasure "if they then promise to bring him to a holy temple so he can be buried with the proper blessings and rituals so his soul can find peace" (a load of BS). But then he leads them straight into some deadly trap, that he activates directly himself, by pulling a secret lever, and then attacks them while insulting them for being "such stupid naive suckers". Plus 2nd hint the history analysis of their heraldic symbol reveals that that their clan of knights got cursed to undeath because they were super evil.

So the "true" proper way to handle the "two doors puzzle room" is just to directly attack the 3 undead. I can make OTHER puzzles, but the 2 doors one is just so well known in the gaming community, it is better to use it as a red herring lol. Basically the big lesson is "Don't thrust monsters!"