Optical Illusion

The figure below, copied form a web site, is quite interesting. As you could get it instant, it's an optical illusion. Look at the picture for sometime (a few seconds..). If you feel headache or if you start vomiting, please excuse me. That's what some of the optical illusion pics do.




















Afterall, its just a matrix of a small geometrical figure. But why the problem ? It's more than just an illusion !