Wow, I could actually follow that explanation, and it's been decades since I thought much about mechanics. Very intuitive.

However, I'm left a little confused at the end, by this:

> The process then repeats itself (imagine a marble rolling frictionlessly between two equally tall hills, starting from a position very close to the peak of one of the hills).

But it's not really just two hills, I think. It must be two hills in a closed space, because you can get from one peak to the other in either direction. Yes?

Also, back in physical reality, I wonder whether the periodic axis rotation continues in the same direction, once a small perturbation gets it started. Or whether the direction of axis rotation randomly changes. Anyone know?