This amazing Pendulum Wave video clip can totally transfix even the most hyper or disinterested class.
You can discuss what is happening on so many different levels. For students who don't believe it's real (some are absolutely certain it's special effects or computer generated), suggest they focus on just one pendulum.
Yesterday one of my students cottoned on to my game : "Are you trying to hypnotise us sir?" I smiled, and suggested she return to focusing on one of the pendulums.