Why does the pitch of a mug change when stirred

Question

I was making some hot chocolate and mixing the mug quite thoroughly, and noticed that the pitch of my spoon knocking against the mug changed according with how fast I was stirring. The pitch was deeper while stirring and moving higher as the liquid slowed and stopped moving in the mug.

In: 7

Answer 1

The pitch can also vary if there’s a handle on the cup depending where you tap on it.