This is the most epic story I've ever heard about a father explaining to his son where babies come from...
"Daddy, where do babies come from?"
"Well, when a mummy and daddy want a baby, they prey to God. And God hear's their prayers coming up the prayer-a-phone, so he tells the angels to go and get some of the white stuff clouds are made of and make it into the shape of a little baby. And then God breaths on them, and theres a beautiful baby"
"But how does it get down here?" Asks the son
"Ahh well" The father started "the angels have special pockets in their coats(?!) and they put the babies in the special pockets and fly down to earth. And when the mummy and daddy are sleeping they put the baby in the bed with them, and when they wake in the morning they have a beautiful baby of their own. And that's how you got here, and how your two sisters got here"
The son thought about this for a moment, then said,
"Oh, so screwings just for fun?!"
(Credit to Mike Harding)