Menno's quote of what Tom Lehrer said reminded me of what Paul Stookey (I think it was Paul) said many years ago in a Peter Paul and Mary concert, when introducing a song where he thought the audience was going to want to join in. I'll try and remember the words.

....and a little voice inside your head is going to say "sing -- sing -- SING!" - and you know, you're REALLY gonna wanna sing! ........PLEASE DON'T!!