This is even better than the collision of shapes implied in the title. Tadashi Tokieda passes a round coaster through a square hole that is much smaller than the coaster's diameter! The simple explanation is that he folded the paper in a way that made a slot as long as two sides of the square. He has a more detailed explanation in this video from Numberphile. This will make a great parlor trick to impress your kids. -via Damn Interesting