better still:
a very young girl was presented with her baby brother. when the time came for nappy changing, Little Girl was very interested in the proceedings, and watched it all. then she pointed to her brother's penis and said, "what's that, mummy?", to which came the reply, "that's his willy, darling. little boys have them, and little girls don't have them" (or similar).
"oh," said Little Girl. she then thought about this, quite hard, and came up with, "good job it isn't on his face."