Most people can see this optical illusion move and shift if they look at it carefully for a short while.
Explanation
Since we live in a three dimensional world, our mind tries to convert a two dimensional object into a 3D one. In this optical illusion, our mind tries to make sense of these 2D objects. The shading used is completely incongruous. The eyes 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 make sense, but it never seems to work.