If you want to waste an entire afternoon and go crazy, go to this site here. It's a page with links to different optical illusions:
http://www.anomalies-unlimited.com/Illusions.html#
Then, there's this page:
http://www.mantasoft.co.uk/_stuff/recursive.htm
My oldest daughter, Helena, found these this morning and I've been looking at them and getting instant headaches. On the first link, you have to click on the raised blank squares to see the different optical illusions.