 There is a video of a gold ball (made of bronze) that can verticly move up and down a string and stop at will. It is able to do this because inside the small ball are two pulleys. Pulling the two ends of the string that go into the ball make the ball go up and releasing the string lets the ball go down. I assume the pulleys inside the ball are spring loaded like a measuring tape so that if you are not pulling the string it rewinds itself inside the ball and the ball moves down.
Check out the video of the gold ball moving up and down. Source: Grand-Illusions.com | 
