Guys put toilet paper in their underwear because a few drops of pee often escape after they've zipped up, and tissue is the only absorbent thing within reach. Harvard Health explains that a little urine can stay trapped inside after the stream stops, then leak out within a minute.The habit sticks because it half-works. For the first stretch of the afternoon the tissue catches the drops, your pants stay clean, and you get rewarded for a system that's about to fail. By the time it does, you've already filed it under "solved."
It also stays private. The UK's
Bladder & Bowel Community notes that few men admit to dribbling even though a great many deal with it, and that it can affect men of any age. So the same workaround gets reinvented in restroom stalls everywhere, by men who each assume they're the only one.