I observed some years ago that if, having first stirred a mug of hot-chocolate one taps the bottom of the mug from within with a metal teaspoon the note given off rises quite distinctly with each tap. I have not reproduced the same effect with tea or coffee. However, any normal ceramic mug and brand of drinking chocolate seems to produce the effect. What, if any, are the special properties of hot-chocolate that produce this rising note effect?