Little Johnny was out with his…

Little Johnny was out with his grandmother when they came across a couple of dogs mating on the sidewalk.
“What are they doing, grandma?” asked little Johnny.
The grandmother was embarrassed, so she replied, “The dog on top has hurt his paw, and the one underneath is carrying him to the doctor.”
“They’re just like people, aren’t they grandma?” said little Johnny.
“How do you mean?” asked grandma.
“Offer someone a helping hand,” said little Johnny, “and they fuck you everytime!”