Subject: Hot Chocolate and rising pitch when tapping on bottom of cup

If you take a cup of hot chocolate (other hot liquids with some slightly 
increased viscosity over water will exhibit the following property as
well) and tap the bottom of the cup with a spoon you will hear a
gradually rising tone (rising pitch of the taps).  The pitch reaches
a certain point and then stops rising.  Upon even a slight stirring the
pitch of the tapping lowers to its original level and begins to rise
again.  Stir, and then wait a minute, and the pitch level begins at the
higher level.

Any suggestions what causes this pitch rise?  I thought it might be
air encorporated into the liquid, though I don't think so, as even
very gentle stirring leads to the tone drop.  I also thought that it
might be the particles of chocolate settling out, though, again, I don't
think so (there doesn't seem to be time for that, and it works for 
liquids that don't settle (like hot tea and milk).

Interesting!  I look forward to hearing from you!

Sincerely,

Michael Burkley
ex-Polyurethane Research Chemist
Current Pastor of the Niagara Presbyterian Church

Re:Hot Chocolate and rising pitch when tapping on bottom of cup