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.
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…