Boys have a penis, and girls have a vagina
Mark followed me into the bathroom, as usual.
Mark: "You don't stand up to pee?"
Me: "No, I don't."
Mark: "Can I see your penis?"
Me: "I don't have a penis."
Mark "You DO have a penis."
Me: "No, I don't."
Mark: "Do you have a penis on your butt?"
I can't stop laughing, but really don't want to talk about my vagina with him, because he will probably mention it to a random stranger at some point.
I finally say "Girls don't have a penis, they have a vagina."
Mark: "You live in China?"