<P> The song was written by Mack Gordon , lyrics , and Harry Warren , music . The song is lip - synched by Lynn Bari in the movie Sun Valley Serenade . Pat Friday sang the vocals with John Payne and The Modernaires . The single , RCA Bluebird B - 11230 - A , reached no . 18 on the Billboard pop singles chart in a one - week chart run . The single release featured vocals by Paula Kelly and The Four Modernaires . The B side of the single was `` Chattanooga Choo Choo '' , which de facto , was treated as the A side . </P>
<P> The song was written by Mack Gordon , lyrics , and Harry Warren , music . The song is lip - synched by Lynn Bari in the movie Sun Valley Serenade . Pat Friday sang the vocals with John Payne and The Modernaires . The single , RCA Bluebird B - 11230 - A , reached no . 18 on the Billboard pop singles chart in a one - week chart run . The single release featured vocals by Paula Kelly and The Four Modernaires . The B side of the single was `` Chattanooga Choo Choo '' , which de facto , was treated as the A side . </P>
who wrote i know why and so do you
who wrote i know why and so do you
Mack Gordon Harry Warren
Mack Gordon Harry Warren