If you haven't checked out Tour de Flex I suggest you do.  It is an Air app that contains over 200 working applications, all with source code.  Everything from simple Flex components to advanced filters and working with Air is covered, so there's something for everyone, beginners to advanced developers alike.

I'm a big fan of optical illusions.  I love how your eyes and your mind don't always agree on things, and this is a great illustration of that. 

In the picture below, square A and square B are the exact same color. You may find this hard to believe; I still do when I look at it, but it's true.  To prove it, take any image program or a colorpicker, like ColorZilla, and compare the colors.  ColorZilla shows that both cells are the color #787878.  Furthermore, I took this image into PhotoShop, cut out square B and overlapped it over square A... guess what? They match.  Click here to see the results.

Can you get your eyes to see the truth?