Most people can see the optical illusion move and shift if they look at it for a while. Many people also report seeing the center section shimmer and shake.
Explanation
The world we live in is three dimensional. We try to convert a two dimensional object into a 3D one. Here, in this optical illusion, our mind tries to make sense of these 2D objects. The shading used in this drawing is completely incongruous. We lock onto a bit of the illustration that looks appropriately shaded but then find it conflicts with the shading in 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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