Here is one of the coolest optical illusions that I have ever seen. It comes from Dr. Edward Adleson of MIT. There are more illusions on his website.

The squares named ‘A’ and ‘B’ are of the same color.

The grey bars in the picture are of the same color. It is obvious from the places where the bars overlap with the squares ‘A’ and ‘B’, that the squares and the bars are of the identical shade of grey. If you still don’t believe this, download the image, open it with your favorite image processing software and check the values for the color properties that the software shows. They are the same: ‘120, 120, 120’.

Via Prashant Mullick