An Old Married Couple

An ancient old couple were sitting in their rockers on their front porch, reflecting on life. Suddenly, the woman gets up and smacks the old man so hard he tumbles off the porch onto the ground. "What was that for?" he asked.
"That's for having such a tiny penis!" she answered. He gets up, dusts himself off and gets back in his chair. After a while, he gets up and smacks her off the porch and onto the ground. She yells "What was that for?" "That's for knowing the difference!"