Most people can see this optical illusion move and shift if they look at it for a short while.
Explanation
Since we live in a 3D world, our mind tries to convert a 2D object into a 3D one. In this optical illusion, we try to make sense of these 2D objects. The shading used is completely incongruous. The eyes lock onto a part 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 3-D solution that would make the shading illustration make sense, but it never seems to work.
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