Suppose you want to hang a picture by a string that’s attached at two points on the back of the frame. How can you arrange the string on two nails such that the picture will fall if either nail is removed?
One solution is above. I don’t know who first asked the question; I first saw it in Mathematical Mind-Benders, by Peter Winkler, who got it from Giulio Genovese, a mathematical graduate student at Dartmouth, who’d seen it in more than one source in Europe.
But it opens up a surprisingly rich discussion — see the paper below for some entertainingly complex variants.
(Erik D. Demaine et al., “Picture-Hanging Puzzles,” Theory of Computing Systems 54:4 , 531-550.)