My Daughter Told Me a Mother/Daughter Joke

This is the joke Sienna told me today:

A mother told her daughter, “Don’t play on a swing when you wear a skirt.”

The daughter asked “Why.”

The mother explained, “You don’t want other people to see your underwear.”

The next day, the girl came home and said, “I played on the swing today. Nobody saw my underwear.”

The mother was puzzled, “How?”

The girl said, “I took my underwear off, Mommy.”