One thing I love about having cool friends like psychologist Richard Wiseman: I see all kinds of things I never would have otherwise. On his blog Richard posted one of the best optical illusions I have ever seen:
Oh baby! Let your eyes roam over it; the squares appear to form a spiral pattern. But then more carefully trace over the pattern -- block some of it with your hand to be sure -- and you see they are aligned in perfect circles.
Still not sure? Let me prove it: all I did was draw in circles.
Even with the circles right on top of them it's hard to see the squares laid out in anything other than spirals.
I love illusions like this, ones that persist even after you can see the trick. And as always, I like to point out that when someone swears they saw something and then say "Seeing is believing," you can point them to things like this and say, "Not always." Remember: it's easy to be fooled, especially when you're not expecting it.