A fairly inquisitive student came up with the following question, and I thought I'd get the reaction of the list as to the best answer.
The course is "Liberal Arts Physics" -- designed as a survey course for those LA majors that need one semester of a science. The theme was energy conservation (a bit broad, and I would narrow it a bit next time I do it).
We are discussing waves. The student asked the following (more or less)
If a wave can transit energy that is proportional to its amplitude squared, and energy can't be created or destroyed, what happens to the energy when two waves destructively interfere?