Timesphere Clock

I'm very fond of technological gadgets that appear to run on the principle of "ooh, magic!" That's why this Timesphere Clock intrigues me. It looks almost like a regular clock, except it has no hands! Instead, a little black marble rolls around the clock at the same rate as an hour hand, indicating the time. Why doesn't the marble tumble off the clock face? Why does it move? How does this thing work? Ooh, magic. $48 USD. (Thanks, Paul.)