This old couple are rocking away on their porch when the woman slowly stands
up and shuffles over to her husband and slaps him across the face, to which
he asked, "What the hell was that for?"

"50 years of mediocre sex" she replied.

He thinks a moment, then works his way out of his rocker, unsteadily walks
with his cane over to where she is seated and slaps her across the face.

"What's that for?" she asked.

"That's for knowing the difference!"