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Facts, science and history of optical illusions.
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How do optical illusions work?

How many pipes?

Source/credit: Griffin_Claw

This is a geometrical illusion where it is hard to tell how many pipes are in the images. Even though vision is three-dimensional, we sometimes perceive the wrong information. There are 2 pipes in this photo, but the shadow of the left pipe creates the illusion of the existence of a 3rd pipe.