Wetting our knickers
When we were younger, me and my sister used to play together in our garden all the time, and this being quite a few years ago would always be in dresses. If one of us needed to wee while we were outside, instead of dashing in for the toilet the sister who needed to go would say "Me wet" then crouch down and wet in her knickers, making sure the other sister would see. The other sister would usually reply "Me wet too" and then squat down and also wet her knickers, and then we would go back to playing while we dried off. I remember it feeling very naughty as the wee poured through my knickers and made a puddle on the ground. I don't know if my sister still wets herself, but I do sometimes!