To tell time, the circle slowly turns as the red/yellow ball rests on the ledge indicating the time. The ball slowly progresses through the numbers until it reaches the 12th hour, at which point it rolls through the hole in the middle and comes out at the bottom again. The clock is about 13 3/4". I love their fresh and creative take on a very old design. This, to me, is an awesome example of form following function.
Here's their website for more information.