Knowing the Difference

An old married couple were driving down the road one day when suddenly the woman punched her husband right in the face. He shouted, "what the hell was that for?". She replied, "That was for 50 years of the worst sex I ever had!"
As they continued down the road, suddenly the man hit his wife square in the face. She turned to him and said, "what was that for?"
He said, "that, was for knowing the difference.