Coffer Ambiguous Figure
Take a look at the image to the right. Can you see the 16 squares aligning horizontally? The vast majority of people see squares, however can you see any other shapes? If you look closer, after a brief period of time, your brain's interpretation of the Coffer Illusion may "flip," giving you the impression of 16 circles. Still cant see them? Perhaps turn the image 90° on it side...
This is the Coffer Ambiguous Figure Illusion, it is one of a series of optical illusions which takes advantage of your ancient heuristic in your mind. It turns out, our brain's have a strong preference for primarily identifying horizontal lines and corners opposed to vertical lines and round shapes. It is thought that this is a canalization of the optical perception based upon the fact that the brain looks automatically for horizontal lines as it aids navigation according to the horizon.