Most people can see the optical illusion move and shift if they look at it carefully.
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 in this drawing is completely incongruous. The eyes lock onto a bit of the illustration that looks appropriately shaded but then finds 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.
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