Little Johnny and his father a…

Little Johnny and his father are observing a couple of dogs screwing each other.
“Dad, what’re the dogs doing?” asks Little Johnny.
“Well, the one below has relaxed and the one above has concentrated.”
“Okay, I’ve understood.”
“What’ve you understood!?” asks the father sarcastically.
“Never relax in your life, dad, or you’ll get fucked like a dog!”