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LSD simulate
it does work if you really stare at the word yo in the middle for like 2 minutes. then look at your wall and shit, with the lights on and the wall gets liquidized
http://www.maj.com/gallery/ShadowMailliw/Custom-Made/magicyo.gif
If i remember correctly optical illusions like this one have to do with either how your brain deals with different lighting and trying to separating the objects differently, or the different density of photoreceptors in the center compared to the further out in. I haven't read up on neuroscience lately so don't flame me if i remember wrong.
after thought: If you really want to learn about things like this a good book to read is Mind Hacks.