… When suddenly, the wife looks at her husband and whacks him across the head, she goes back to her book.

Her husband, puzzled, asks, “What was that for?”

She replied, “That was for 50 years of bad sex.”

They both go back to their books, and a few minutes later, the husband looks at his wife and whacks her across the head, he goes back to reading his book.

The wife, also puzzled asks him, “What was that for?”

Not looking up from his book the husband answers, “That is for knowing the difference.”