Quotes / Crowd Song

Rocko: Why was everyone singing?
Heffer: We just got a song in our hearts!
Rocko: How is it you all know the words? Did you rehearse?
Heffer: Yeah, every Thursday. Didn't you see the flyers?
Rocko's Modern Life, "Zanzibar"

Helen: It was a very convincing song, Marge!
Skinner: Mm-hm, there were kicks and everything.
Marge: Oh. Can you sing it again?
Ned: I'm sorry, it really was one of those spur-of-the-moment type things.
The Simpsons, "Bart After Dark"

940. I will not abuse the Exemplary virtue to set up highly choreographed dance routines with random crowds.
1425 Things Mr. Welch can no Longer Do in an RPG

Robert: He knows the song, too? ...How do they know this? I've never heard this song before!

How did they all know the words? Was there a rehearsal I was unaware of?