https://www.youtube.com/watch?v=krD4hdGv...

The coolest things sound waves do.
https://www.youtube.com/watch?v=Ude8pPja...

When I worked at the Hands-On Museum in Ann Arbor, we had something less complicated. They put steel plates on long pins and gave the public bows - violin bows but made of metal handles for durability - the public being what it is... You draw the bow across the plate and watch the sand dance in standing waves.

When I was demonstrating how to make fireworks from your medicine cabinet and kitchen sink, I referred to the steel plates. We perceive the resonance of photons within the electron shells as colors. In fact, electrons themselves are resonant standing waves.

The resonance frequency of sand is a key to ProjectX.