The hot chocolate effect, also known as the allassonic effect, is a phenomenon of wave mechanics first documented in 1980 by Frank Crawford, where the pitch heard from tapping a cup of hot liquid rises after the addition of a soluble powder. It was first observed in the making of hot chocolate or instant coffee, but also occurs in other situations such as adding salt to supersaturated hot water or cold beer. Recent research has found many more substances which create the effect, even in initially non-supersaturated liquids.
Surprising phenomenon that I was unaware of. Via the excellent blog FYFD