The Newton disc

HOW

1. Look at the stationary disc.
2. Then gradually rotate it so that it spins around fast. At the right spinning speed, you will see a uniformly coloured, grey-white wheel.

WHY

The image that enters the human eye is not captured instantaneously. If the rotation speed of the disc is high, the different colours on the disc reach the eye so quickly that it cannot distinguish them. As a result, all colours are interpreted as a single signal.