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When we look at a very bright object and then look away from it, why do we still see the object’s outline?

Difficulty: Tricky Scarlett, Azure and Jett were drinking tea together. There was one scarlet red teacup, one azure blue mug and a jet-black tea glass. None of them had a cup that was the same colour as their own name. After a while, Scarlett offered to get the person with the black glass another drink….