Quae nocent docent

rifmeister posts a wonderful little problem here, and it reminded me of an entirely different wacko problem posed on the xkcd balrog bolgia blog "blag". I don't particularly recommend slogging through all the comments, as a reasonable number of them are useless or worse, but the link to the Wikipedia article on the "two envelopes problem" is certainly worth a look.

I bet some of you want to know how to crochet a hyperbolic plane.