A few strange things going on here. Sit back in your seat so you are not too close to the screen. Then look directly at the center green square. Firstly the sides of the different squares start to deform, secondly a ghostly circle appears around the green square. And, by the way, which green square is the biggest?
Original image source: unknown - Animation by Optical Spy
Animated by Optical Spy from an unknown source image
If you keep you eye on one of the balls the other one seems to be all over the place. Then try it with the other ball.
Optical Illusion Website
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