Magical spiral disk
Spin the disk in a clockwise direction and focus on the center for around 20 seconds. Afterwards, look at your palm and observe what happens.This phenomenon is known as the waterfall-effect. When focusing on the spinning disk for a couple of seconds it appears as if you were looking into a rotating tunnel. If you look at your palm directly afterwards the spinning image continues to have an effect on the retina. Hence, it seems as if your palm was moving in the opposite direction, curving towards you.
Further informationson the exhibit "Magical spiral disk"
If you spin the disk counter clockwise it seems as if your hand was curving inwards.