Ask:
"which road would the other two point me?"
> You find the one who speaks the truth; his answer is: my lying companion will send you either of these paths and the other one will stab you.
>You find the lying on; he will say it's either one of the wrong two paths and tell me to ask the stabbing guard because trust me, i'm a guard.
>You find the stabbing one, for he will try to stab you (RUN!!!)
The idea is:
you are stuck in a room, there are 2 exits, 1 is heaven and 1 is hell, you don't know which one is witch, there are also 2 people, who who always tells the truth, and knows everything (including which doors are witch), and one who knows everything (knows the doors also) and will always lie. you are allowed to ask any 1 question, if you ask 2, they kill you.
Ask one what the other would say to being asked if his door leads to heaven. If he says "he would say his door leads to heaven" then the person you asked's door does. Because if he's the liar, the other guy wouldn't really say that, and if he's telling the truth, the other person is lying about his door being the right one.
You would ask one of them which door the other person would send me through then go to the opposite. As the one that lies would send you to the wrong one and the one that tells the truth would also tell you which one the lying one would send you Heard this one before.....
"Raise your hand if you're the one who stabs people who ask tricky questions."
Stabby will raise his hand. You did not ask a question so you won't be stabbed.
The liar will raise his hand as well.
You just found the truth teller.
******* XKCD, I tried to appear in one of those comics once. Apparently, Physicist-Murdering cuttlefish are funny, but try to talk about how to turn aborted fetuses into Semtex, and everybody's all "Go back to Cyanide and Happiness" and ****