Perception

Ah, the age old quote ' perception is reality'

Here's a couple of web sites with great illusions that show how our eyes can decieve us.

eChalk.co.uk - Colour Perception

PatMedia - Cool Illusion

Regarding the second one, I have it on good authority (my mom) that it works better if you shut one eye :-) Oh.... and apparently if you stare at it as long as mom did, you'll get a headache :-(

I love these sorts of tricks - they can be provide a great intro when challenging peoples' opinions about a subject, something I've had to do in the day job on a few occasions...