I discovered a couple of interesting blogs that specialize in optical illusions. Click on the above art so that you have only the art, and not my entire blog, on your monitor. Now, stare at the centre black cross. The colour should change to green and even begin to disappear. If it doesn't work try moving closer --- I was about a foot from my monitor when it worked. If you like this, check out some of the other optical illusions on the Mighty Optical Illusions site.