Before entering the topic, let's type 'Necker Cube' to the google search engine.  Then you can see following result.

 

 

 

If you are a quick witted, you already know what it is.

 

The Necker cube is kind of an optical illusion named by Louis Albert Necker (1786-1861). It is a simple wire-frame drawing of a cube, but interpreted differently for different users. People could perceive either the cube as a simple 2D image, 3D cube with the lower-left square as a frontal face, or 3D cube with the right-upper square as a frontal face.  

 

 

Posted by Cat.IanKang
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