What is the answer to this riddle?

Does he have to ask the same question of each guard, or can he ask different questions?

If you are not limited to the same question, the answer is trivial. Ask one guard "What is 1+1?"

If the first guard answers anything but 2, then you know the other guard is going to tell you the truth, so you ask him "Which door is the one that leads to safety?"

If the first guard answers 2, then you know the other guard is going to tell you a lie, so now you are screwed, and you are best to just stay a prisoner for now rather than take a 50-50 chance of dying.

he doesn't actually need to ask both of them

he asks: "If I would ask your brother, what would he say which door lead to safety"

Both answers will be the same and safe path is another one

One that tells truth will think "right door is A /for example/ - my brother lies so he would say "B", but I am saying truth, so my answer is that my brother would say - "B"" and he tells "B"

One that lies thinks: "right door is A, my brother tells truth so he would say "A", but since I am lying my answer is that he would say "B"" and he also tells "B"

They will both provide same answer and door that leads to safety is other one.

That's not necessary MOU. You don't need to ask him anything about his brother. Putting the question in the subjunctive case is what makes the question work. You can ask one guard, "If I asked you which door to take would you say 'yes'?" The subjunctive mood creates an additional item to lie about so the lying guard gives a doubly negative answer, which makes it positive.

Not quite. the riddle states the lying guard will lie, but since he is a guard and does not want the prisoner to get out, he will lie in such a way as to make the answer not only a lie but also useless to the prisoner.

So when asked "If I would ask your brother, what would he say which door lead to safety?", the guard knows his brother will point to the correct door, so a lie would be "My brother would say you should eat a mushroom". This satisfies the demand that the anwer be a lie, and also not provide the prisoner with any useful information.

The truthfull guard, when asked this question, knows that the lying brother can say anything he wants as an answer to this question, therefore the truthfull guard is forced to answer the question as "I don't know what the other guard would say."

Nice twist, Ped. We assumed the traditional answer, and retrogressively assumed the riddle should fit the answer. We decieved ourselves and you alone are correct. Expanding on your point, the truth-telling guard need not respond to the question he was given. He may simply respond to any question by saying, "I luvs me mum!" Also, we don't know if the guards know more than we do. No matter how you phrase the question the honest guard may simply tell you he has no idea. Or even offer his opinion, "I would go that way."
It does appear the best solution is to stay put. That is assuming he is not suicidal, or a werewolf. In the latter case he probably asked his friends not to let him out no matter what, and they wouldn't give up their positions regardless of how he phrases his one question. Why would we even assume the guards would let him through either door? The honest guard is likely honorable and doesn't take his job lightly, while the liar probably delights in the torment of his captive.

MOU was right, ask which way the other would tell you to go. Then go to the other door.

Sorry, Individual, I don't see how that would work.

Example: you ask the liar "Which door would the other guard tell me to use?" The liar now thinks "My brother, since he always tells the truth, would say to use door A which is the Door of Safety. Therefore, it would be a lie to say that my brother would say to eat a mushroom." So now the liar will respond to your question by saying "My brother would say to eat a mushroom." So you are still stuck.

pedro - if liar always lie and that's your claim in first post then he is always providing wrong answers. period. his brother would answer "A" and liar will say "B". if he is making answers up to confuse you more he is not liar. so, clear that first

scoates - yeah, I know, but that's too complicated Especially in Croatian

Lie: to create a false or misleading impression

I don't see how telling him to eat a mushroom could lead him to the wrong room.

Of course, eating the mushroom could lead to a hallucinogenic state that would possibly make him stumble into the wrong room.

yeah, I am not sure in english terms and phrases, but in croatian langauge lie is lie, exactly as Individual makes definition
Saying "he would say to you to eat mushrooms" if you ask him about doors is just nonsense. Not lie.

but, actually, if you insist, you said he can ask each guard same question. So, "liar" can say "mushroom" if he wants, truth teller will still tell you "A" if "B" is correct answer and "B" if "A" is correct answer.
So, you in that case have to choose between "A" and "mushroom". Still you win.

If you can ask only ONE guard one question then riddle cannot be solved, because "liar" can always answer you "mushroom"

You could just kick each guard in the crotch, then peer into the room to see what lies on the other side.

There, that's the answer to your riddle.

Or THREATEN to beat up either guard. Even the liar may insunuate the truth if he felt his life was on the line.

But then you don't have the element of surprise, and two against one is anything but fair.

I doubt the two guards care too much for each other. They have conflicting personailities. Before they capture us as their prisoner they likely got bored--severely and often--and discussed politics and religion, and so forth. Anyway, chances are they talk about something to pass the time, and chances are they don't agree with each other. So here's my plan, your one question shoudl be "Is lying good?" The honest guard will make himself known by saying that it is indeed not. Then he'll start talking to the dishonest guard trying to get him to change his ways. Something they have often discussed. This really annoys the dishonest guard who can't stand his associates "constant preaching." You join in until the two of you have made the dishonest guard very angry, then you suddenly turn on the honest guard and kick him down. Sensing an ally the dishonest guard will join in. After you each have a few kicks suggest the two of you go for a drink. Open a door for him, and if it's the wrong one he won't want to go through it. You see, you've tricked him so you can judge his actions rather than his words. Just because he's a liar doesn't mean he has a death wish.

P.S.- If he doesn't drink, say "Oh, neither do I, I just want a soda."

Are we allowed a lifeline on the question?

this is confusing!!