OK, more Fabdom: This '63 release was dedicated to the band's many adoring female fans. B-side to "From Me To You". Lennon wrote the verses and Macca the chorus. First Beatles song to utilize double-tracked vocals to emphasize John's lead (he had a bad cold that day!). Two other songs, "One After 909" and "What Goes On?", were recorded during the same session but were not released until much later - "Let It Be" and "Rubber Soul" respectively.
Yesterday's answer: John's Aunt Mimi always asked him which guitar part was his on Beatles recordings - he tuned the 4th string slightly flat so she could tell which one was out of tune. The other "unofficial" reason was to annoy the much more obsessive/compulsive Paul.