A father and son walk out onto their porch overlooking the stream and the father puts his arm around his son, stares at the scenery, and asks:

Dad: Son, do you have any questions for me about sex?
Boy: Yes as a matter of fact I do. What's that hairy patch below Mommy's waist, in between her legs?
Dad: That's her vulva.
Boy: Oh... Well, what's a cunt?
Dad: That's the rest of Mommy.