Why does a mug of hot chocolate sing when you tap it?
The hot chocolate effect
Why tapping a mug of hot chocolate makes a pitch that rises as you stir. It’s the bubbles.
Stir hot chocolate powder into milk and tap the mug with a spoon. The note starts low and climbs over a few seconds.
The stirring fills the drink with tiny air bubbles, which slow down sound. As the bubbles escape, sound speeds up and the pitch goes up.
Your mug is a musical instrument. Go annoy someone.