Brother, can you spare a penis?

Strapped in her carseat, in the back of the car, with her little sister strapped next to her, Sadie suddenly and proudly proclaims, "I don't have a penis!"
"That's right," I said. "What do you have?"
She grins, "A vaginis."
I tell her that boys have penises and girls have vagin-NAHs. I asked her, what does mommy have?
"A penis," she responds. "Is mommy a boy or a girl?" I ask, trying to lead her to the right answer by using the Socratic method. "A girl, " she says.
So what does mommy have? I ask again. "A vaginis," she replies. This goes on for few rounds and everyone I ask about seems to have a penis to her, her grandmothers, father and friends at school.
Finally, we go back to square one because she says in a rising questioning tone, "I don't have a penis?!"