I have recently been wrestling with this little problem: I take a hot cup of powdered chocolate and stir it. I then tap the bottom of the cup repeatedly. The sound continues to increases in frequency/pitch as I continue tapping at a constant speed. After I stir the liquid, the sound returns to a low frequency. Tapping again creates the steady increase in frequency. The question is, what is happening in the cup with the waves and particles created by the tapping that increases the frequency of the sound? Keep in mind that tapping the outside of the cup delivers the same result... I am stumped, being a physics moron. :) Help anyone?