I always heard it told as: Little Johnny overheard his older brother talking about "pussy" and "bitches", but he doesn't know what the words mean. He goes to his mother and asks her what 'pussy' means, and she replies, "Son, a pussy is a cat". He then asks her, "Well what is a 'bitch'?" and she replies "a bitch is a female dog". Johnny is confused by the answers. They don't make sense in the context he heard he brother. He decides to ask his dad, to be sure. "Dad, what is a pussy?" Johnny asks. His dad is busy watching football and just reaches into the drawer beside him, pulls out a Playboy magazine, and draws a big black circle around the centre-fold's crotch and says "That, Son, is a pussy." Johnny then asks, "Well what's a bitch then?", and Dad replies "it's everything outside the circle." (I know it's degenerate on multiple levels, so take it or leave it - it's a joke)