[thedoc (OP)]

Sherry asked the Naked Scientists:
   Why, when I stir my cup of coffee, does the spoon strike different notes when tapped on the side of the cup as the swirling motion slows down?
I'm not sure if it's relevant but the coffee is made with just full milk.
What do you think?

[chiralSPO]

I believe this has to do with how much of the cup is in contact with the liquid inside. When the coffee is swirling, it becomes shallower in the center and deeper at the edges, and the faster it swirls the more this difference is (even forming a vortex in extreme cases). So the faster it swirls, the more of the cup is touching the liquid, and the way that the cup vibrates is altered.

This effect will probably be most pronounced when it is filled to about a third to a half way.