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