A elderly couple are sitting on their porch in their rocking chairs, watching the world go by when suddenly the old lady reaches over and smacks her husband upside the head so hard he falls out of his chair onto the porch.
He slowly stands back up, rubbing his back. "What the hell was that for?" he asks.
"For 47 years of terrible sex," comes the answer. The old man stares at his wife for a moment, then shakes his head and slowly climbing back into his chair to resume his rocking.
A few minutes later, the old man's arm suddenly shoots out, smacking the old lady so hard she tumbles out of the chair and all the way down the porch steps.
"And just what the hell what was THAT for?" she demands.
"For knowing the difference."