Is the center circle at left larger than the center circle at right? How about this: are there gray spots between the corners of black squares?
Illusions are tricks that play off of the ways our brains typically process sensory information. The problem is that many of them have become clichéd. Hence illusion illusions — illusions that play off of the illusions we’re used to seeing. See the angry-comment-provoking Flickr set.