An old couple are sitting on their porch when the wife looks at her husband and smacks him on his leg as hard as she can.
"Ouch!!" he says. "What was that for?"
"That's for 41 years of horribly boring sex!"
He falls silent and they resume looking at the view for a few minutes. Then the husband looks at his wife and hits her on her leg as hard as he can.
"Ouch!" she cries. "What was THAT for?"
"That was for knowing the difference!"