Philip's theory
Yes, it's the Mexican and his hat. I wonder how many IB physics students have puzzled over this picture in the past. Unfortunately he's not in the new edition. Decided to go a little lighter on the humour. Several years ago a Mexican student promised to give me his hat when he left. A couple of days after he left I saw another student wearing said hat, apparently there had been a final change of allegiance. The hat still gets passed around but one day it will be mine.
When dealing with the difference between a standing wave and a progressive wave I use the examples of a skipping rope and a Mexican wave. A Mexican wave is what Brits call a stadium wave. However one of my students Philip informed me today that this is wrong. A Mexican wave is a standing wave (It's caused by standing).