Little Mary was out with her grandmother when they came across a couple of dogs having sex on the sidewalk.

"What are they doing, Grandma?" asked the little girl.

The grandmother was embarrassed, so she said, "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?"

"How do you mean?" asked the grandmother.

"Offer someone a helping hand," said Mary, "and they screw you every time!"