Slinky

I'm sure you, dear readers, being more intelligent than most, are familiar with slinkys. You may think of them as kid's toys, but they were way, way more.

Aside from watching them walk down stairs on their own, we used them in high school science class to demonstrate wave action.  We would all gather in the hall, stretch one out, and simply whack one end.  The energy would move a wave of compressions and expansions all along the 20 feet of the slinky. Wave action learned in a moment.

Toy indeed.