> I tried it again with hot chocolate, and it didn't work because the foam on top was too thick (maybe a millimeter). I got rid of most of the foam, and then it started working again.
>
> More evidence for the friction theory.

I just tried it with a cup of fake cappucino. There was foam in a ring around the outside edge of the mug, while the top of the liquid had an ultra-thin layer of the stuff. The outside edge of foam did not move in relation to me or the mug, but the inside layer of foam DID appear to move as I rotated in my chair. I think the friction theory is quickly on its way to becoming proven fact.