Eureka!

Two cavemen are sitting on a hilltop one sunny friday afternoon, bashing rocks with stone hammers. One has chipped a flat rock into a rough disc, and stands the disc on edge to put the finishing touches. Letting go for a moment to reach for a smaller hammer, he's amazed to see the stone roll down the hill, gathering speed. They look at one another in disbelief, then each grabs a flat rock and bashes it into a disc. They stand the stones on edge and watch them roll down the hill. Again they stare at one another with raised eyebrows and repeat the process. A cavewoman who's been watching all this approaches them, and says "If you made them square they wouldn't roll away!"