"What would the result be of combining the following terms with "and": the direction of the correct door, and the color of the sky?"

Right, in which case the door they're in front of is the safe door. And if they tell the truth and say yes, they're still the person in front of the safe door. By asking it that way they make it so it doesn't matter if they're the liar or not. "Yes" means that person's door is safe and "No" means you want the other door, no matter who you ask.

You can ask both guards if an item is an item. "Does this cup contain fluid" would work, it doesn't have to be a dead guy.

no because the question was which door would the other person say is safe, they both point to the not safe door

I wish our DM had real-life message to telepathically convey stuff to just one person. In my group there would be literal zero chance of the others not listening to me if I ever threw a “hmm why is that wire there”, because they would’ve heard the dm either tell me due to passive perception or had me throw a roll and then tell me. So they know it’s a trap no matter if I want to rp it. Every time I get frustrated and question it, there’s thistle guy who always has the reasoning and justifying at hand why they would know to do the right thing and to be fair they kind of make sense always, but there’s zero chance he’d come up with that just by my rp line alone without knowing for a fact it’s a trap. I think that’s the worst kind of meta gaming. They are fully blind to the meta gaming there and just do it by instinct. And when you try and question, they always have a defense ready, even if it’s so wildly specific and unlikely but you can’t really fault it because they’re not stupid, the justifications hold, it’s just that the only way they habitually generate them is because they know what I know despite they couldn’t in-game know. Like I’ve occasionally just left the thing unsaid in-game out of frustration and just reason to DM that there’s so much going on, my focus instantly switched to another thing and I forgot because I’m not very smart. So we all know there’s a trap but now nobody has told this to the others. What do they do? The one guy fucking always comes up with some shit like “hmmm be wary, they must’ve laid traps here, hey you with good investigation, please look around and see if there’s one in this specific place for some reason” and the rolls of course often succeed because they always choose to best one to solve that. But from rp perspective, we’ve walked this path for a while, and this thought only came up now, that it might be trapped? Just right now when you know, outside of the game, that there’s a trap? I call bullshit and it frustrates me so much, there’s very little chance of anything interesting ever happening in-game because we seldom miss anything or do the wrong things, because “somehow” we always happen to do the right things no matter who notices things in-game or rolls or whatever, no matter how much any of us attempt to rp it, somebody just meta games it without it being explicitly or admittedly meta gaming and gets all defensive when questioned and because they now know everything, can figure out an explanation the DM can do nothing but accept because it makes sense, now that they know to pull the right shit out their ass. Ugh. It’s not even a big deal, our group is fun and the adventuring isn’t bad, these things don’t happen often enough for it to really affect things, but god do I hate it. This ended up being a rant, I didn’t even know how much I get frustrated with it until I just now read this back. Jesus…

JAJAJAJAJAJJAJAJAJAJJAJAJAJAJAJAJJAJAJAJJAJAJJAJAJJAJAJAJAJJ almighty

Well, obviously. But a barbarian might have a preference.

I think you mean HEY, WHAT'S UP?

You should even specify “path to the castle”, because there isn’t _technically_ a “correct” path.

No because them pointing at a door is answering a different question than the one that was posited in the question.

yeah, it could be the liar guard's desire or prin to send you down the deadly path. to him that could be interpretated as the correct path. especially if these are automatons working off of some machine logic. like, they don't even need to be out to get you, that's totally something that bad code could do on accident.

The liar responds "I don't know"

And the surviving guard will most definitely answer a 2nd question despite the rules.

This puzzle was used in more than one place than in Labyrinth. I played video games where they had that puzzle (Ultima 6 had that).

The question we ask if "What would the other guard say if I asked him which door is the good one?" Liar says Bad Door Truther says Bad Door Now, for their answers to update, they would have to ne answering the question, "Which door would the other guard say if I asked him 'Which guard would the other guard say is the good door?'" We want a guard to answer "What would the other guard say is the good door?" Regardless of how they answer our "outer" question, the answer to the "inner" question ("which is the good door?") doesn't change. Liar doesn't care that Truther would say that "Liar would say the right door is the good one," Liar is being asked how Truther would answer "Which door is the good one". I know I basically just said the same thing three times. My brain isn't working to break this out the elegant way I can't quite assemble. But hopefully some part of all this helps. The crux is that the question that they are imagining the other guard's answer to is not the same question they themselves are being asked.

What is the quest was to die asap. And everyone the party meets just refused to kill them?