Older Couple

An old couple were sitting on the porch one afternoon rocking in their rocking chairs. All the sudden the old man reaches over and slaps his wife.
She says, “Well what was that for?”
He says, “Thats for 40 years of rotten sex!”
She doesn’t reply and they start rocking again.
All the sudden the old lady reaches up and slaps her husband.
He says, “Well what was that for?”
She says, “That’s for knowing the difference!”