Why Quantum Paths Cancel

The story

Quarky sets up two shiny slides and lets a tiny particle race down both at the same time. When the particle arrives, some spots on the wall light up bright and other spots stay totally dark. Quarky discovers that each path carries a little wave, and when two waves arrive out of step they cancel each other out, like two kids on a seesaw pushing at exactly the wrong moment. That is why nature paints stripes of light and shadow instead of one big blur.