Around 15:30 into the song, Roger Waters says: "Who was trained not to spit - in a fan." Although it is probably a coincidence, Waters once spit at a fan. Pink Floyd performed at a concert in Montreal, and when a fan tried to climb onto stage, Waters hocked a loogy into the guy's face. This gave Roger the idea for the movie and album The Wall
-in an interview he said that the event created a barrier or a "wall" between them and the fans.
Chuck - Los Angeles, CA