This evening, in the bath:

RM: Do you know that boys and girls have different private parts?
C: Yes, boys have butts.
RM: Well, girls have butts too, but where did you hear that word?
C: From Mrs. [Preschool Teacher].
RM: Hmm. Well, we call a butt a bottom and it's in the back.
C: Girls have vulvas in the front.
RM: Yes, and boys have penises.
C: I want a penis.
RM: You do?
C: Yes, someday when I'm older I won't be C anymore and I'll have a penis.
RM: Well, no, you'll always be a girl your whole life, so you'll never have a penis, er, probably.
C: But I want one.
RM: Well, if you had one, then you'd never be able to have a baby.
C: Oh. Then I don't want one.

Baby trumps penis! Take that, Sigmund!