What was that for?

An elderly couple was sitting on a two person porch swing one fine summer afternoon. Out of nowhere, the woman smacks the man on the shoulder.
"What was that for?" asked the man. "That was for 50 years of bad sex!" came the reply.
Five minutes or so later, the man smacks the woman on the shoulder.
"Ouch, what was that for?" asked the woman.
"That was for knowin'!" came the reply.