Steganography is the art of hiding something in something else in plain sight. Usually images or text are hidden within other images or sound files. For example, in the image below of trees there is an image of a cat hidden inside it. Wikipedia explains that for each component of each RGB value, if you take just the last 2 bits of it and then turn the brightness up 85%, you get a picture of the cat. The whole point is so the image of the trees looks identical to an image of the trees without an image hidden inside to the human eye.
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