Each eye sees a slightly different world. (Put a finger in front of your face, switch from one eye open to the other and that finger will shift, just a little bit.) But rather than walk around all day seeing in double vision, your brain pulls the world back into one-ness.
Brains decide what we see. Kokichi Sugihara knows this better than anyone. He makes videos that trick your brain into seeing things that you know, you absolutely know, can't happen.
And yet —
Obviously wooden balls cannot run up a slope. Yet they seem to. Sugihara uses no editing tricks. His props are cardboard and glue, no special effects. All he does is find the precise angle where our brain assumes an impossible act. In this case, he chooses the angle that makes down-sloping planes look like up-sloping planes.
Then he changes the angle and you see how he did it. (Or more correctly, how you did it.)
Brains, you may not realize, make arbitrary assumptions to keep our world intact. Sugihara knows exactly where those assumptions pop into place. Using simple paper and glue constructions, he creates shapes cannily designed to make us see something that isn't happening. A "Specially Appointed Professor," Sugihara teaches at the Meiji Institute for Advanced Study of Mathematical Sciences in Japan, and in his longer video "Impossible 2" he did this:
...and a whole lot more. If you have 13 minutes to waste on insanely ridiculous illusions, Kokichi's your man.
Special thanks to Carl Zimmer for turning me on to this. Professor Sugihara won the Vision Sciences Society's Best Illusion of the Year award in 2010 and if you want to make what he made, here are some building instructions.