A man and his wife of more than 50 years were rocking back and forth on the front porch. Slowly they rocked, in rhythm as this was their time to spend a few quiet moments and after years of practice they rocked to the samepace.
Suddenly the wife stopped, grabbed her cane, and with a loud and hard WHACK hit her husband across the shins.
His eyes watered and tears ran down his cheeks. When he finally caught his breath he gasped and asked, "What'd you do that fer?""
That's fer fifty years of bad sex," she said.

He nodded his head, but said nothing. Slowly they began to rock again. Again they kept pace. Back and forth, back and forth they rocked, until suddenly the man stopped, and picked up his cane. He reached over and with a loud, sharp WHACK, he hit his wife across the shins.
As soon as her eyes quit watering and she could speak she asked, "What was that fer?""
That," said her husband as he began to rock again, "is fer knowin' the difference."