The path was blocked by two guards. "One of us always tells the truth and the other always lies," they said. "You may only ask one question." "If I asked the other guard if I was valid and worthy," I asked one, "what would they say?" "They would say 'no'." Satisfied, I left.
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If you ask "what will the other guard say", you always get a lie; if you ask the guard who tells the truth, he will truthfully tell you what the lying guard would say, if you ask the lying guard, he will lie about what the truthful guard says.https://xkcd.com/246/
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(Normally the aim is just to get past the guards, e.g. https://tardis.fandom.com/wiki/Osiran_service_robot …, but the choice of question is what makes
@MicroSFF story new.) The are versions with complications like the logician not knowing the words for yes and no in the answer.1 reply 0 retweets 12 likes -
I don't know for certain if Martin Gardner invented the puzzle, but he certainly popularised it. https://blogs.scientificamerican.com/guest-blog/the-top-10-martin-gardner-scientific-american-articles/ … https://wordplay.blogs.nytimes.com/2014/11/10/truth-2/ …https://www.theguardian.com/science/alexs-adventures-in-numberland/2014/oct/21/martin-gardner-mathematical-puzzles-birthday …
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Couldn’t you ask “What’s 1 + 1?”
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Replying to @MockStarNZ @alanrmb and
Usually the point is not to find out which guard lies, but to find out some bit of information the guards know, like "which door leads to safety" or "am I valid and worthy". So asking "what's 1+1" or "are trees plants" is fine, but doesn't solve the problem.
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Notably, you can't find out both pieces of information with one yes/no question - you can ask a question you know the answer to to find out which one lies, or you can ask a question that forces the liar and truth teller to act alike because you actually want the answer
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