First produced during the Ming Dynasty (1368-1644), iron balls have long been famed as one of the "three treasures" of Baoding China. Produced by skilled Chinese craftsmen, iron balls were originally solid but are now hollow. 

Baoding balls are thought to exercise hand muscles, improve brain function and reduce stress when used as alternative medicine to stimulate acupuncture points on the hand. 

The basic exercise consists of holding a air of Baoding balls in the palm of one hand, rotating them (switching the relative position of the two balls) while maintaining constant contact between them. Once this technique has been mastered, the rotation speed can be gradually increased until the balls are separate in the hand. Eventually one hand can learn to completely rotate them without the balls making contact with each other.