have the number 1 or 2 in mind. Is the number that you have in mind larger than the number I have in mind?” If the guard answers “yes” then you know that they are thinking of number 3. If the guard says “I don’t know” then he is thinking of number 2. If the guard says “no” then he is thinking of number 1.paleonaps95 wrote:I think that (the sibling one) may be the only foolproof one, because the guard could say no for two as well as one.
His number is 3: 3 > 2 and 3 > 1 so he answers yes
His number is 1: 1 < 2 and 1 = 1 (1 is not less than 1) so he answers no
His number is 2: 2 = 2 (2 is not less than 2) which would mean no, but 2 > 1 which would mean yes. Because it could be yes or no his response becomes I don't know.
Its foolproof, if that doesn't make sense, which it probably doesn't, someone else has explained this well already:
"If the guard has 2 in his mind, then he can’t answer “No” for certain, since if you’ve said that you have 1 or 2 in your head. If you have 2, then he could say no, but if you have 1 he’d say yes; not knowing which of these is in your head he’d need to say “I don’t know”.."
