4 People
Shifting shape (to see enlarged version, click link.)	Shifting shape (to see enlarged version, click link.)	Shifting shape (to see enlarged version, click link.)

If you take a look at the following  picture , let me tell you ... it is not animated.  Your eyes are making it move.  To test this, stare at one spot for a couple seconds and everything will stop moving.  Or look at the black center of each circle and it will stop moving.  But move your eyes to the next black center and the previous will move after you take your eyes away from it.... 

Click to enlarge

I think it's because we have more experience looking down at a staircase from the top than we spend looking up at the bottom of it from under it.

A woman is driving along a winding road. As she comes up to a blind corner a man in a BMW comes round the corner in the opposite direction, swerving across the road into the oncoming traffic. As he drives past the woman, he shouts through his window "PIG". She retaliates with "ROADHOG" and drives on - round the corner into a pig in the road.

Original drawing	Rolling marble
Floating bubble	Bouncing rubber ball