On a close-up view, you can see on the left face, an angry man and on the right face, a woman with a neutral facial emotion.
But further back (walk some 5 meters back from your computer monitor), the faces change expression and even genders! If you squint, blink, or defocus, an angry man should substitute for the face of the woman and the left angry face should not be angry anymore.
More images like this on Computational Visual Cognition Laboratory @ MIT
(Tip from Marike)