Little kid wakes up in the middle of the night and wants a drink of water. He walks into his parents room while they're having sex. His eyes get really big: "Daddy, Daddy, what are you doing to Mommy?"
"We're making babies, kid, shut up and go back to bed!" He scurries back to bed.
A week or so later, the kid is out walking with his father and they come upon two dogs humping. His eyes get really big: "Daddy, Daddy, what is that one dog doing to the other dog?"
"Don't worry kid, they're just making puppies!"
Later that week, he again wakes up in the middle of the night and wants a drink of water. He walks into his parents room while they're having sex. His eyes get really big: "Daddy, Daddy, what are you doing to Mommy?"
"I told you kid, we're making babies. Shut up and go back to bed!"
"Daddy, Daddy, turn Mommy over, I'd rather have puppies!"