We've been struggling with what to do for the cover art of Java Puzzlers, and some brainstorming brought us to the idea something like the amazing optical illusions of professor Akiyoshi Kitaoka. They are absolutely mind-blowing. At first you'll think they are animated GIF images, but I assure you the pictures move when printed on paper too! Professor Kitaoka has two books in print, and plans to publish English translations soon. In the U.S. you can get the Japanese books from the Kinokuniya bookstore.