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A judge says to a prisoner: "You're going to be hanged at noon on one weekday next week, but the day of the hanging will be a surprise to you, and you will not know which day it's going to happen until the hangman knocks on your door at noon." The prisoner figures out that he will not be hanged. How? The prisoner correctly reasons that he cannot be hanged on Friday, because if Monday-Thursday pass without him being hanged, a Friday hanging will not be a surprise - therefore, it cannot possibly occur on Friday. The prisoner then realizes that if he isn't hanged by Wednesday, the hanging cannot occur on Thursday, because, since Friday has been ruled out, Thursday is the only possible day left - therefore, it cannot possibly occur on Thursday, since it won't be a surprise. The prisoner then realizes that if he isn't hanged by Tuesday, the hanging cannot occur on Wednesday, because, since Thursday has been ruled out, Wednesday is the only possible day left - therefore, it cannot possibly occur on Wednesday, since it won't be a surprise. The prisoner keeps going backwards, and realizes that all of the days are impossible, and that he will not be hanged the following week. On Wednesday at noon, the hangman shows up, knocks on the prisoner's door, and the hanging takes place, much to the prisoner's surprise. --- For anyone who has trouble grasping paradoxes, consider the Liar's Paradox: "Everything I say is a lie." If they're lying, then it's a true statement; but it can't be a true statement since everything they say is a lie. Don't think about it too hard - contradictions will mess with your mind. --- On a different subject, if you *really* want to mess with your mind, try and visualize the beginning of time, or the end of space. Trust me, it's not worth it - it will end up making you dizzy or nauseated (picture a cat trying to understand calculus).
