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There is a level of unpleasantness, like when we hear about the injuries of someone we care about, that causes our ball-sacks to retract in a very unpleasant manner in a response to empathy so strong that our bodies preemptively and uncontrollably protect our most sensitive organ. The visual effect displayed on our faces is likely a response to the unnerving feeling of pelvic muscles forcibly pulling our nuts back to the pelvic bone.