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It's to do with the pressure of the tissue beneath the bellybutton; if someone has a flattie, or a very slight outie, then the tissues behind may either push it into an outie (say for example they breath in, pushing their tummy, and therefore bellybutton out too), or suck it into an innie (say for example they breath out, causing a slight vacuum suction to pull the bellybutton into their tummy). This can be artificially reproduced as well, as all people with true outies can push their outies in, and many people with true innies can pull their bellybuttons out to become an outie

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13y ago

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