An elderly couple were sitting in their living room, each reading a section of the newspaper.  Suddenly, the wife stood up and walked to her husband while rolling up the paper.  She then hit him over the head with it.

"OW!  WHAT THE HELL WAS THAT FOR?!"

"That was for 50 years of bad sex!"

The wife returned to her chair and resumed reading.  Minutes later, the husband rolled up his paper, casually walked over to his wife and proceeded to hit her over the head.  

"That's for knowing the difference."