effect, he and McCartney had a shared purpose. Author David Rowley points out that at least half of all Lennon–McCartney lyrics have the words "you" and/or "your" in the first line. In Lennon's 1980 Playboy interview, he said of the partnership: Historian Todd Compton has noted that there is some truth to Lennon's statement regarding McCartney's optimism. However, it does not tell the whole story, as some of McCartney's most characteristic songs are tragic, or express themes of isolation, such as "Yesterday", "She's Leaving Home", "Eleanor Rigby" or "For No One". Although Lennon and McCartney often wrote independently—and many Beatles songs are primarily the work of one or the other—it was rare that a song would be completed without some input from both writers. In many instances, one writer would sketch an idea or a song fragment and take it to the other to finish or improve; in some cases, two incomplete songs or song ideas that each had worked on individually would be combined into a complete song. Often one of the pair would add a middle eight or bridge section to the other's verse and chorus. George Martin attributed the high quality of their songwriting to the friendly rivalry between the two. This approach of the Lennon–McCartney songwriting team—with elements of competitiveness and mutual inspiration as well as straightforward collaboration and creative merging of musical ideas—is often cited as a key reason for the Beatles' innovation and popular success. As time went on, the songs increasingly became the work of one writer or the other, often with the partner offering up only a few words or an alternative chord. "A Day in the Life" is a well-known example of a later Beatles song that includes substantial contributions by both Lennon and McCartney, where a separate song fragment by McCartney ("Woke up, fell out of bed, dragged a comb across my head ...") was used to flesh out the middle of Lennon's composition ("I read the news today, oh boy ..."). "Hey Jude" is another example of a later McCartney song that had input from Lennon: while auditioning the song for Lennon, when McCartney came to the lyric "the movement you need is on your shoulder", McCartney assured Lennon that he would change the line—which McCartney felt was nonsensical—as soon as he could come up with a better lyric. Lennon advised McCartney to leave that line alone, saying it