The Private Ear is the ninth episode of Season 3 of The Brady Bunch, and the 58th overall episode of the series. Written by Michael Morris and directed by Hal Cooper, it first aired 12 November 1971 on ABC.
Peter uses his father's tape recorder to snoop on his siblings' conversations, creating dissension in the household, so Greg and Marcia decide to teach him a lesson.
Marcia isn't talking to Jan because Marcia told her a secret which she told to you know who. Greg isn't talking to Marcia because Greg told her a secret which she told to you know who. Bobby isn't talking to Cindy because Bobby told her a secret which she told to you know who. When Mike and Carol find out that no one seems to be talking to anyone else, they come to the quick realization that the common "you know who" is Peter. They learn directly from the horse's mouth that Peter, just as a joke, has been bugging the boys' and girls' bedrooms with Mike's tape recorder.
Mike counsels Peter on his behavior and Peter apologizes to his siblings. Mike considers the matter settled, but deep down, Greg and Marcia think Peter will keep on doing what he's been doing since his punishment was no punishment at all. So they decide to test their theory by planting a message about a surprise party for Peter on the tape recorder to see if he listens to it. Mike and Carol learn of this and sabotage the revenge by throwing him an actual surprise party.