Most people will see this optical illusion moving and shifting as they look at it for a short while.
Explanation
We live in a three dimensional world. Our mind tries to convert a two dimensional object into a 3D one. Here in this optical illusion, we try to make sense of these 2D objects. The shading used is completely incongruous. We lock onto a bit of the illustration that looks appropriately shaded and then find it conflicting with the shading in other nearby areas. We keep jumping around the illustration trying to map out a 3D solution that would make the shading make sense, but it never seems to work.