The response to the question of why the sound of spoon hitting the side of a mug increases in pitch as the coffee is stirred struck me as being a bit more convoluted that it needs to be, and probably just plain wrong. Think about what happens when you stir a mug of coffee: as you begin stirring, a small vortex appears, small at first, just a dimple in the surface of the liquid. But as you stir more vigorously, this vortex becomes larger and deeper. Where does all the liquid go? Up the sides, of course. Tap a glass or mug with some liquid in it. Add some more liquid and tap it again. Hear that? Enough said.