Thanks Chris but I'm afraid your answer doesn't quite explain what I've observed. Perhaps I didn't give enough detail.

1. First stir the hot chocolate (remember this doesn't work with tea).

2. Start tapping down through the drink onto the bottom of the mug - you will hear a satisfying 'plink' sound - continue to tap (but not too hard or you'll knock the bottom out of the mug with painful consequences).

3. Listen to the note of each 'plink'. Each one will be higher than the last as you continue to tap.

4. Stir the chocolate again and the 'plink' returns to its original lower note and once again rises with each tap (which is what makes me think it isn't a fall in temperature Chris.)

5. When the chocolate reaches optimal drinking temperature sit back and enjoy (without marsh mellows - nasty habit!)

Come on folks this has got my science colleagues stuck - there must be an explanation. My hunch is it's something to do with cocoa particles in suspension being flung outwards by centrifugal force (thus making the centre of the drink less dense), and then drifting back to be more evenly spread throughout, (making the whole drink more evenly dense) but... ...I dunno...I'm a bit dense myself! ;-)

I don't think so because, as I say, the rising change in pitch can be recreated by stirring the chocolate again. Stir and the pitch goes down and gradually rises again. Also, as I say, I don't get the same with tea or other hot drinks I've tried (not tried them all to be fair).

The stirring/chocolate seems to be significant! Anyone? Tom Knight

Interesting. However, if the change was due to the chocolate disolving then  wouldn't one expect the effect to stop after the first few stirs (as the chocolate would presumably be disolved so no more disolving = no more change = no more rising pitch)? This is not the case. One can keep recreating the effect each time one stirs the chocolate (until you get bored and decide to drink it.) And I'm pretty sure it won't work with tea or coffee (2 lumps or 0).

You have try this one at home!