Johnny and his father were taking a walk, when Johnny noticed two dogs mating.

Johnny asks his father, "What are those dogs doing?"

"Well they are making puppies," his dad says.

The next day they see the same dogs mating and already Johnny says, "Hey they are making puppies!"

That night Johnny walks in on his parents having sex. Curious, Johnny asks, "What are you two doing?"

"Well we are making you a baby sister," said his dad.

"No no no" yells Johnny, "roll her over and make me some puppies!"