I'm a big fan of optical illusions.  I love how your eyes and your mind don't always agree on things, and this is a great illustration of that. 

In the picture below, square A and square B are the exact same color. You may find this hard to believe; I still do when I look at it, but it's true.  To prove it, take any image program or a colorpicker, like ColorZilla, and compare the colors.  ColorZilla shows that both cells are the color #787878.  Furthermore, I took this image into PhotoShop, cut out square B and overlapped it over square A... guess what? They match.  Click here to see the results.

Can you get your eyes to see the truth?