I found these optical illusions to be really mind blowing. The first is the Coffer Illusion. Stare at the rectangles long enough and you may see, well, different shapes…circles.
The second, I swear to you, is not a GIF. It is your brain trying to make sense of ambiguous shapes causing motion. Our brains just don’t show us an object as it is, but rather our brains try to make sense of WHAT an object is.