According to journalist Bill Harry, McCartney wrote the lyrics while shaving, but McCartney told biographer Barry Miles that he wrote them while on a tour bus. He also said, "It was the first song I'd ever written the words first. I never wrote words first, it was always some kind of accompaniment. I've hardly ever done it since either." The lyrics follow the "letter song" model as used on "P.S. I Love You", the B-side of their first single.