An old couple, high school sweethearts married for 60 years, rocking on the porch twiddling their canes. She reaches over and cracks him on the shins.
"WHAT THE HECK IS THAT FOR?"
"That's for 60 years of bad sex."
A couple of minutes later he cracks her on the shins.
"WHAT IS THAT FOR??"
"That's for knowing the difference."