Look in your belly button at the end of the day, and you might find a loosely packed tuft of soft lint. For many people, belly button lint is a normal fact of life.
But take a second to consider it and it might seem more mysterious. Where does the lint come from? Why do some people get it, but other people don't? And how the heck does it form, day after day?
These questions have probably puzzled people since shirts were invented hundreds of thousands of years ago.
Luckily, as New Scientist has reported, exactly one scientist has taken the time to study this topic in detail, and he believes he has the answers. It all has to do with stomach hair.
How belly button lint forms
Georg Steinhauser of the Vienna University of Technology is primarily an atomic chemist. However, in 2009, he published the results of four years of research on belly button lint (or as the Brits charmingly call it, "navel fluff") in the journal Medical Hypotheses.
By studying a total of 503 pieces of lint from his own belly button — and asking friends about their navel fluff production patterns — he came to a simple conclusion. Stomach hair is responsible for producing belly button lint.
"The scaly structure of hair firstly enhances the abrasion of minuscule fibers from the shirt and secondly directs the lint into one direction — the navel — where it accumulates," he wrote. "The hairs’ scales act like a kind of 'barbed hooks.'"
Steinhauser first arrived at this hypothesis by finding that of his friends, it was mostly people that had stomach hair who also typically found belly button lint. He proved it by shaving his own stomach, and seeing that he didn't produce any belly button lint until his hair grew back.
He also confirmed the seemingly obvious fact that lint originates from shirt fibers in two ways: by seeing that it always matched the color of the shirt he was wearing, and by chemically analyzing the lint and finding that it was mostly made of cellulose (the material that makes up cotton). It also contained some nitrogen and sulfur, likely from sweat and skin cells.
Finally, Steinhauser made one more interesting finding. By weighing every single one of his navel lint balls and noting the condition of the shirt he was wearing each day, he found that new shirts produce way more belly button lint than old ones, probably because they have more loose fibers to give off.
As you can see, the vast majority of his lint balls weighed less than three milligrams. However, a few brand-new shirts produced relatively huge lint balls, as heavy as six or seven milligrams.
This means, Steinhauser calculated, that "Assuming that an average T-shirt (180 g) can be worn up to 100 days a year, such a shirt could suffer a weight loss of up to 182 mg (0.1 percent) per year just by fibers that are lost into the navel."