This little old couple, been married forever, are sitting on the couch one evening watching tv just like they've done every night for the past 30 years. All of a sudden the little old lady turns and punches her husband in the arm really hard. "Ow!" he says, rubbing his arm, "What was that for?"
"That's for 30 years of lousy sex," she says.
They continue watching tv in silence. The man turns and punches his wife in the arm. "What was that for?" she asks.
"That's for knowing the difference."