Messing your diapers for fun -
A long time ago (at least 55 years ago), I wanted to be a baby once again. I knew babies wqore diapers and did their pee and poop in the diapers. Alas, I was an only child, so there weren't diapers around for me to steal and wear. One day, I decided, I can pretend my underpants are a diaper and pee in them.