Grandpa was showing little Johnny around the farm, and when they came to the corral they saw two cows having sex. He explained, "That's a bull and a cow, and he's serving her."

A little later on, they saw horses fucking. The Grandpa said, "That's a stud and a mare, and he's serving her, too."

That night at supper, after everyone was seated and grace was said, Grandma turned to Grandpa and said, "Will you please serve the turkey?"
Little Johnny jumped up and yelled, "If he does, I'm eating a fucking hamburger!"