I found this linked from one of the comments on a Slashdot story about optical illusions. I quite like optical illusions - especially ones which I can twist in by brain and "break". This one I couldn't do that to though. I even cut some of one square out and moved it around. As I moved it closer to the darker one the piece I was moving seemed to darken and vice versa as it moved to the lighter one. Totally Freaky!