Rounding a rock - GIF

[Bony]

I've seen spherical granite about the size used as an ornament in Western Australia, the rock rests in a concave spherical "dish" of the same diameter (well almost) into which a supply of water is introduced. When the water flows the sphere turns, almost without friction and will continue to rotate randomly for a very long time. The ball can be stopped by hand, with difficulty, then will slowly commence to rotate. It's fascinating to watch, and will keep kids amused for what seems like ages. I was certainly intrigued for a while. Can dig out photos if anyone is interested, but I don't think I have video.