A little twist applied to a novelty neon flower bulb. These usually screw in directly into a common AC light socket. In this case, however, there is a small circuit that powers them with a squarewave source. The lamp is connected to an "H-bridge", and the squarewave'd duty cycle is varied. When the duty cycle is 50%, both flowers are equally illuminated. At 90% duty cycle, one flower is fully lit, while the other is very dim. At 10% duty cycle, the reverse is true. Slowly sweeping the duty cycle (10% to 90%, back to 10% ...) makes the flowers fade in and out as shown.