Just attended a nice talk by Erik Demaine on Origami and puzzles. He demonstrated some remarkable stuff and ended with a cute picture hanging puzzle – how to hang a puzzle with two nails such that by removing either one, the picture drops. A brief description is at the end of the following paper
http://erikdemaine.org/papers/FUN2004i_TheoryComputSys/paper.pdf
and a preprint dated 2004 that seem to be still unavailable is announced. I would love to have a look at that illusive paper.
In case you haven’t seen the preprint;
http://arxiv.org/abs/1203.3602
Enjoy