This illusion has been around for a while, and we were recently reminded of it again by Chris Stevenson, from London.
Hard as it may be to believe, the two squares A and B are actually the same colour. There are different ways of proving this, and we have seen some elegant solutions.
Edward H Adelson is Professor of Vision Science at MIT, and is the creator of this illusion. If you want to know how it works, you will have to visit his site...