Granddad and Grandma were sitting at the table. Grandma get up suddenly and rolls up her newspaper and proceeds to slap Grandad over the head. He says, "What was that for?"
Grandma says, "That's for 40 years of bad sex."
Grandad sits there muttering, then he rolls up his newspaper, and goes over and slaps Grandma around the head. She says, "Now what the hell was that for?" He says,
"That's for knowing the difference."