Why does the pitch of tapping with a teaspoon rise in a mug of hot milk?
Need: Ceramic mug (shape doesn't matter), hot semi-skimmed or full-fat milk, metal teaspoon.
When I give the milk a good stir, then begin to tap the bottom of the mug (inside) with the spoon, the pitch of tapping rises gradually. When I stir the milk thoroughly & then start tapping again, the pitch resumes its ascent from the original lowest note. If I only give a partial stir, the pitch resumes partway up from the lowest note. What's going on?