The checkerboard optical illusion...some people will only believe this is real when they print it out and physically cut out the piece and move it, or bring it into a paint program and "lift" the color and move it. Believe it or not, this isn't a joke, it is real! Two major reasons this illusion works is the human mind does a good job at "ignoring" shadow-like effects. This helps humans not be fooled by shadows. The other major contributor to the illusion is the surrounding of pieces by light in one case, and dark in the other. These combined cause your mind to tell you what you are seeing isn't possible! Enjoy! I've just added the picture to this text and again I find it impossible to believe it is true, but I assure you it is!
And if you liked that one, here are some more! Click on them to enlarge the image. I have found a great website with illusions, check it out!