Ive always loved optical illusions. Its as if they find the brains blind spot, its weakness.
This one for example:
If looked at in the right spot, theres no doubt that its a sphere. but i bet that if you look at it from directly above, it might look like a sphere but there would be no shadow.
concentrating on the top half of the picture the corner points away from you, the bottom it points toward you
a lot of optical illusions can be found in street art:
but my favorites have got to be the chasms