Two drunk guys stumble out the back of a bar to go pee in the alley. While their both facing the wall the one guy looks back over his shoulder and sees an old mangy dog licking its private areas.
So he taps the other guy on his shoulder and says, "Hey check out what that dog is doing."
The second guy says, "Haha, yeah I sure wish I could do that!"
The first guy slurs back, "You should probably just pet him first."