There have been a lot of unique designs 3D printed, but this has to be one of the coolest yet. David Hagemann has created an optical illusion that shows a protruding face whether or not the face is inverted or not. The illusion is based on conditioning of our minds, and very few people can actually see the inversion. Read and see more on this incredible illusion at: http://3dprint.com/53071/3d-printed-optical-illusion/