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This question already has an answer here:

You are on a trail in a desert. You already drank all your water, and are 2 miles from the next well.
There is a crossroads in the trail. One path leads directly to the fountain, and one leads out into the middle of nowhere. At the crossroad, there is an Indian. He is from one of two tribes - one tribe always tells the truth, and one tribe always tells lies. The two tribes have one thing in common: They will only answer one question.

What question must you ask to find out which trail to take?

I'll post the answer in a few days if no one knows it.

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marked as duplicate by Rob Watts, Gilles 'SO- stop being evil', Doorknob Oct 11 '14 at 23:57

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Pretty vanilla knights and knaves riddle. You should ask

"If I asked you which road leads to the fountain, which would you say?"

Assume A is the correct road, and B is the wrong road.

The truth-teller would normally answer A if you asked which road is safe, so when you ask which road he'd say if you asked which road is safe, he'll still say A.

The liar would lie, and tell you that B is the safe road, but since he has to lie to answer your question, he tells you that A is the safe road.

You may still die from thirst before you get there, of course.

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  • $\begingroup$ It's only two miles - you won't die. :) $\endgroup$ – Cullub Oct 8 '14 at 19:49
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    $\begingroup$ OK, you might get jumped by bandits. Or a piano may fall on your head. In a universe where you instinctively just know you're speaking to someone who is either as honest as they come or a pathological liar, many things go unexplained :-) $\endgroup$ – Joe Oct 8 '14 at 19:50
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"If I asked the representative from the other tribe, "Which road leads to the fountain?", which would HE say?"

Assume A is the correct road, and B is the wrong road.

The truth-teller would normally answer A if asked about the road directly, but would know the liar would say 'B' as his response for the fountain road.

The liar would lie always, and say the truth teller would say B is the safe road.

As such, you know the wrong direction is B from either Indian, and you should choose A.

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"What would you say *yesterday* if I asked you, 'Which is the right way'"

  • Yesterday the liar (he who always tells lies) would tell a lie (= the wrong answer). Today he must tell a lie on that lie, so you get the real answer. (lie + lie = truth)

  • Yesterday the honest person (who always only tells the truth) would tell... the truth (you guessed it! :) ). Today, he must tell the truth on the truth, so you still get the real answer. (truth + truth = truth)

In math:

Liar: -1 * -1 = 1
Honest man: 1 * 1 = 1

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