The large granite ball weighs about half a ton, but you can move it using only one finger!
The granite ball is an accurate sphere which sits in a granite cup. A water supply in the base of the exhibit can be pumped into the bottom of the cup. When the pump is switched on, the gap between the ball and the cup fills with water. At quite a modest pressure (about 3 pounds per square inch), the ball is lifted clear from the cup and can therefore be rotated easily.