Spin it under a strobe light and you get the optical illusion of marbles rolling around! At different speeds it will show like the marbels are going forward, backwards, or standing still.  You'll want to install a strobe light app on your phone, or for an additional $5 I'll send you a viewer that acts like a shutter.  But I don't recomend that, it doesn't work nearly as well.  I'll provide a link to an app.