This is just adorable for young voices, with such an optimistic message!  It's Bobby McFerrin's big hit, which won the Grammy in 1989, and it's just as much fun as ever.  Be sure to use the recorded accompaniment track - it is sensational.  Complete with finger snaps, "Doo doo doo's" and optional parts for two flutes, this is not to be missed!  Highly recommended!