One of my favorites

For your enjoyment, allow me to present one of my favorite optical illusions of all time. In the picture, the squares marked "A" and "B" are the exact same color and luminosity. Placing them next to each other makes them indistinguishable.

I enjoy this picture because it is proof of the stubborness of my own perceptions. When I first saw this image, I didn't believe the two patches were the same color. I loaded the image in a program and literally dragged the first panel next to the second. Identical. I still didn't quite believe it, so I printed it out and cut them out and placed them next to each other. The same.
To this day, when I look at the original image, I still don't BELIEVE they're the same color, even though I know and have confirmed that it's true. Every time, I get the urge to go through the proof again, just to quiet my mind. No doubt there's some deep psychological principle at work here....