A most excellent optical illusion

Yes, I woke up early this morning, 6am-ish, and did my Web research on medical informatics. So now I'm taking a break, waiting for Fedex to deliver my Dragon NaturallySpeaking Medical software.

Here's a most excellent optical illusion created by a MIT professor, and served up on BoingBoing.net. Squares A and B are the same gray tone, despite what you're perceiving. I brought the image into Macromedia Fireworks (now Adobe), and sliced out a portion of the A square to bring it along side square B. Voila!: