This old couple was sitting in their
 rocking chairs on the back porch when
 the old lady reached over and knocked
 the old man out of his chair.

 The old man got up, sat back down in
 his chair and said "What was that for?"

 The old lady said "That was for 50
 years of bad sex."

 A couple minutes later the old man
 reached over and knocked the old lady
 out of her chair.

 She got up and said "What was that for?"

 "That's for knowing the difference."