I love optical illusions, and I'm fascinated by the mechanics of vision, so I have to share with you this video. This technique of animation has been known for a long time, but it's still pretty cool.
Another video discusses how this is done. I had a card I carried in my wallet for years that did a similar type of illusion using a lenticular overlay which, when you moved the card back and forth, made it look like little colored spirals were rotating in different directions. I bought it when I was in college, but sadly I lost it last year (I'm pretty sure I dropped it at Comic Con). I have no idea how to replace it. If any of you has seen something like it, please let me know!
Tip o' the Fresnel lens to BABLoggee Cristiana Senni.