John Lennon wrote this at a transcendental meditation camp in India when he couldn't sleep. He was meditating day and night, and after 3 weeks of meditation and lectures by Indian gurus he missed his soon-to-be wife, Yoko Ono, and came up with the song. The Beatles went on the retreat to study with the Maharishi. When it was over, Lennon thought it was a crock, but he wrote some good songs while he was there. (thanks, Vicky - LA, CA)
The voice at the end sounds like, "Paul is dead man, miss him," when played backward. This helped fuel rumors that McCartney was dead and replaced by an actor that looked like him.
The line "When I hold you in my arms, and feel my finger on your trigger" from " Happiness Is A Warm Gun" appears in bootlegged, unreleased versions of this song as "When I hold you in your arms, when you show each one of your charms, I wonder should I get up and go to the funny farm." (thanks, Conrad - Los Angeles, CA)
The word "get" as used in this song is a well-known term as a quite mild insult that is still commonly used on Merseyside. Lennon is cursing Sir Walter Raleigh (who is credited with introducing tobacco to Britain from America in the 16th century) for indirectly getting him hooked on cigarettes. (thanks, Steve - Liverpool, United States)