If you watched Saturday morning cartoons during the seventies and early eighties you probably saw the "Schoolhouse Rock" cartoons. They featured catchy little songs which skillfully obscured the fact they were teaching useful lessons along the lines of "how a Bill becomes a law" or "A Noun is a person, place or thing." The original TV series of some forty little ditties won four Emmy Awards. The series evidently struck a chord with many young viewers, because the it has since spawned a CD with covers of the songs done by Gen-X bands and a stageshow extremely popular with, you guessed it, twenty-somethings. The touring show in Minneapolis tonight at the Music Box Theatre. Lorna Benson asked George Newall, the creative director of the original cartoon series, how Schoolhouse Rock came about.