My granddad was an inventor. One time he had a long slinky (I think he must have joined a few together) that was hanging, stretched out parallel to the floor, on a number of closely spaced fishing lines, attached to a 4x4 overhead.

At one end he had a geared down variable-speed motor that pushed the slinky with a sine wave motion. I believe the other end was fixed. In between, he had painted various parts of the slinky blue. The blue parts seemed to be completely still, even as the areas between the blue pulsated back and forth.

He said it was a demonstration he built "to show my ninkompoop investors about standing waves".