This is a fun joke that will especially amuse married couples.
An elderly husband and wife are sitting on the front porch after their fiftieth anniversary party. Suddenly, the wife leans over and punches her husband hard on the shoulder, and says, "That's for fifty years of bad sex!" Well, the husband thinks this over for a minute, then leans over and punches her right back in the shoulder, and says, "That's for knowing the difference!" Not bad, huh?