Take a look at the objects below. Now look again. What do you see? Most likely you perceive this as an "impossible object." Your brain is trying desperately to make this shape "work" as a three dimensional figure, even though it wouldn't be possible in the physical world.
Our brains love viewing figures as three dimensional so much that it persists in doing so even when the figure doesn't make sense. Even though the brain knows that the object breaks the rules of physical possibility, it keeps perceiving it as a 3-D object instead of a 2-D collection of lines and shapes.