On the morning of their fiftieth wedding anniversary a couple was eating breakfast together. He was reading a newspaper, she flipping the pages of a magazine.
The wife suddenly jumped up from the table, tightly rolled her magazine and began beating the man about his head and shoulders. The attack was notable for its ferocity. Finally, spent, exhausted, and out of breath, she collapsed in her chair. She fixed her husband a look of malevolence and told him "That is for fifty years of bad sex".
The man slumped in his chair. He was battered and bruised. Blood seeped from small cuts on his scalp. His glasses were askew. He stared at his wife in shock. He slowly stood. He rolled up his newspaper and proceeded to attack his wife with reciprocated fierceness until she too was beaten bloody.
"That", he told her, "Is for knowing the difference".