Can you hang a painting from two pegs, so that if you remove EITHER of the pegs, the painting will fall down?
Let's see if this accepts HTML
<img href="http://home.comcast.net/~owenkl/Sketches/TwoPegs.gif">
or BBCode
[img=http://home.comcast.net/~owenkl/Sketches/TwoPegs.gif][/img]
This solution (if it shows) is topographically identical with Erik O.'s, except that the pegs ar placed vertically instead of horizontally.