The other day I found some Baoding balls in the cupboard. These are the balls which people rotate in the hand. Here's a demo video of how an experienced user rotates them. His balls also make a sound during rotating:
I have simple solid marble balls, so they don't produce a sound which I really don't mind, because the "music" in the above clip got a bit annoying after a while. According to traditional Chinese medicine the balls interact with acupuncture points on the hand during rotating which invigorates the brain eliminates worry, can prevent hand tremors, etc. So you do a service for your body with some balling.
The basic exercise is rotating a pair of balls in the palm of the hand, so that they are in contact constantly. On advanced levels the balls should be rotated so fast they don't touch at all. In both directions. Currently, I always hit one of the balls with other, so even the constant contact is beyond my abilities.
When one is doing something physically passive (watching TV, phoning, etc.) then it's a good time for some practice, and it's quite addictive too. It's also useful when thinking about some problem, because the rotating balls occupy the body, so the mind can focus better at the task at hand.
I recommend trying it out. It's fun.