South Park (1997)

Season 1: Episode 9

South Park Elementary is on stage rehearsing for the schools play. Kyle begins to sing a song about Mr. Hankey, the Christmas Poo. Of course, no one has any idea what the heck he's singing about. Kyle explains that Mr. Hankey comes every year to all the good boys and girls who have a lot of fiber in their diet. Everyone thinks he's insane and he gets sent to a mental institute.