The dance of the diamonds
You probably haven't heard of the Chladni effect but you've likely seen it in action. Sprinkle some grains of sand on a thin metal plate and play a violin bow across it, and you'll notice that the grains bounce around for a bit before settling down into a pattern, and refuse to budge after that.
This happens because of a phenomenon called a standing wave. When you drop a rock into a pond, it creates ripples on the surface. These are moving waves taking the rock's kinetic energy away in concentric circles. A standing wave on the other hand (and like its name implies) is a wave that rises and falls in one place instead of moving around.
Such waves are formed when two waves moving in opposite directions bump into each other. For example, in the case of the metal plate, the violin bow sets off a sound wave that travels to the opposite edge of the plate, gets reflected and encounters a newer wave on the way back. When these two waves collide, they create nodes – points where their combined amplitude is lowest – and antinodes – pointes where their combined amplitude is highest.
In 1866, a German physicist named August Kundt designed an instrument, now called a Kundt's tube, to demonstrate standing waves. A short demonstration below from user @starwalkingphoenix:
