I Visited the Science Museum of Virginia in Richmond and stopped by one of its main attractions: The Grand Kugel. The Grand Kugel is a 29 ton granite sphere (kugel is the German word for ball) that is seemingly floating on flowing water. The friction between the sphere and its base is so low that you can move the kugel with your bare hands!
The physics of how this is possible has been studied. The kugel doesn't really "float" on the water, rather the water is pumped under pressure from below the kugel producing an upward force that matches the weight of the rock and creates a lubrication effect. In this sense the Kugel behaves like a big ball bearing. Due to the momentum it gains once it is set in motion, the Kugel can go on for hours before stopping.