Your forehead is so big that NASA went to discover Mars, but then they said, "Oops, wrong planet. Mars is smaller than this, we will discover it later."
NASA Forehead Discovery
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Explanation
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Explain Bear
Hey, dingus! This joke is saying your forehead is so massive that NASA mistook it for Mars. NASA, which stands for National Aeronautics and Space Administration, is the U.S. government agency responsible for science and technology related to air and space.