Moving Nuts Optical Illusion

Can you see how this optical illusion seems to move and shift?

See Another Random Optical Illusion

Scroll down for the explanation.

Most people can see this optical illusion move and shift when they look at it 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 object. In this optical illusion, we try to make sense of these 2D objects. The shading used in is completely incongruous. We lock onto a bit of the illustration that looks appropriately shaded but then find it disjointed with shading in other nearby areas. We keep jumping around the drawing to try to map out a 3D solution that would make the shading illustration make sense, but it never seems to work.