What makes you a boy?

This exchange took place between my 2 1/2-year-old son and I while driving.

Son: "Daddy, you're a boy and a daddy?"

Me: "That's right.  And you're a boy and a son."

Son: "What makes you a daddy?"

Me: "You make me a daddy.  Having kids makes me a daddy."

Son:  "I make you a boy too?"

Me: "No sweetheart, we're boys just because."

Son:  "What makes you a boy?"

Me:  "Well, boys have a pee-pee.  Girls don't have a pee-pee"

Son is silent for 10 or 15 seconds.

Son:  "Girls only poo-poo?"

I die inside from joy and amazement.

Me:  "No, I'm sorry, I wasn't clear.  Girls do go Pee-pee, but they don't have a penis like you do."

Son: "Girls only have butts."

Me:  "Well, the do have a butt just like you, but they have different parts.  They don't have a pee-pee, they have different parts."

Son:  "Girls have different parts?"

Me:  "That's right."

Son: "My parts are better?"

Me: "No sweetheart.  Girl parts are different.  Yours are not better, just different."