Impossible Box 3D Optical Illusion

Why couldn't you actually build this?

See Another Random Optical Illusion

Explanation

This optical illusion makes use of the techniques of overlap, perspective angles and shading.

Some of the parts of this object are either in front or behind where they should be. Additionally, the perspective angles at some corners are wrong. The shading causes your eye to travel along an edge and group together parts which, logically, wouldn't go together.