Making a baby

Mom and Dad were taking young Billy for a walk through the park one sunny afternoon when all of a sudden, in the bushes a short distance away, Billy spots two dogs going at it. Billy says, "Daddy, what are they doing?"
The dad responds after some quick thinking, "Why son, they are making a puppy."
Later that night Billy was thirsty and got out of bed to get a glass of water. As he walked by his mom and dad's room, he heard a noise and looked in only to find them going at it. Billy shouts, "Daddy what are you doing?"
The father, quite embarrassed, replies "Why Billy, we're making a baby."
"Quick, turn her over..." declares Billy, "...I want a puppy!"