Hot Chocolate Experiment

Have you ever stirred your hot chocolate and noticed that the pitch increases as you continue to tap?  If you haven't, listen here: <WAV file of pitch changing while tapping>.  Some people think it's annoying but I think it's curious.  Why does the pitch change?  One person thought I must have changed where I held the spoon (my fingers were slipping), but that's not true; the spoon is held in the same position.  I started to think about this in a scientific manner and put some hypotheses together.

Here are recordings of the tapping sound under different conditions.  Each set of conditions tests a different hypothesis. 

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