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

Coffee

Source/credit: racingtothevioid

There are two lamps above this COFFEE sign. One on the left is closer and casts a little stronger shadow of the letters COF. The one on the right casts a shadow of the letters FEE. The two shadows of the letters FF overlap on the wall, creating the word COFEE.