Well probably the occasion when I led 30 odd people, definately not musicians, in an african boat song and managed to get them not to strain their voices but to sing. The night just went on and on and I know from the reaponse the day after that it was a night many of then would never forget. The power of the human voice when it's not under strain and played in unison may yet be the richest of instruments - and every one can do it.