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 same pace.
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."