Guy King wrote

What I can't find is the explanation for why the sound of tapping a
spoon on the bottom of a fresh poured drink rises in pitch as you
continue to tap. I saw one a while ago, but I can't remember the
details. Something about microbubbles changing the speed of sound in the
liquid but that doesn't always seem to fit what's happening.

When you pour a hot liquid into a (relatively) cool cup, the cup will
expand. If you're stirring it, it will be ringing like a bell and a
bigger bell means a...

Oh. Oh, well.

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-blj-