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xkcd #2869: Puzzles
(imgs.xkcd.com)
Alt text:
Why couldn't the amulet have been hidden by Aunt Alice, who understands modern key exchange algorithms?
I had an elementary school teacher who would do these puzzles with our small class.
It was much better than your situation though: he would already know the solution and basically we took turns asking him yes or no questions until we figured it out.