Reduced gravity shallow water waves!

You found my experimental work: a latte macchiato is a great way to see reduced gravity shallow water waves in action. When the espresso is sufficiently hot*, it will form a distinct layer above the warm milk. When you jiggle the glass, observe the lower frequency variations along the coffee-milk interface (where the density difference between layers is slight) in comparison to the higher frequency variations between the coffee and foam layer above (where the density difference is larger).

*My efforts to show this in class have thus far proved unsuccessful. It doesn't work to simply bring the warm milk and espresso in thermos containers. If we could only get the Courant to install a proper espresso machine in the classrooms. (For education purposes, of course!)