Bells have been used in courtrooms for centuries. (TOS: "Court Martial" ; TNG: "Encounter at Farpoint" , "The First Duty" ; DS9: "Rules of Engagement") In the past, teachers used small hand-held bells to signal their students that the lesson would begin. Molly and Miles O'Brien gave Keiko a bell as a present for her first day as a teacher on Deep Space 9. (DS9: "A Man Alone")