The two-nails problem is my favourite nerd-snipe. It can be explained to anyone without a math background, it can be solved on paper, it isn't a "trick" problem

The two-nails problem is my favourite nerd-snipe. It can be explained to anyone without a math background, it can be solved on paper, it isn't a "trick" problem, and it isn't immediately easy. The problem: hang a painting with one unbroken piece of rope going from one corner of the painting to another around two nails such that if either nail is removed the painting will fall.