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A dream, scrambled eggs, a mother, and Julian Lennon -- what do they have in common? They're all things that inspired some of Paul McCartney's best known songs with the Beatles.

"Yesterday" was basically a McCartney solo track, written and recorded by him backed by a string quartet, although it was still credited to "Lennon-McCartney." Initially, the melody for the song came to McCartney in a dream in 1964, which led Macca to believe that he had subconsciously stolen it from someone else.

"I just fell out of bed and it was there," McCartney told Rod Granger of The Hollywood Reporter. "I have a piano by the side of my bed and just got up and played the chords. I thought I must have heard it the night before or something, and spent about three weeks asking all the music people I knew, 'What is this song?' I couldn't believe I'd written it."

Once McCartney was assured that the tune was indeed original, he had problems nailing down the lyrics, and initially — temporarily — titled it "Scrambled Eggs," with the temporary opening couplet, "Scrambled eggs / Oh, my baby how I love your legs." There was also concern that the somber ballad didn't fit with the band's Fab image. Finally, during a trip to Portugal in May 1965, McCartney had a breakthrough and composed the tune's proper lyrics.

Under the guidance of producer George Martin, the track was recorded with McCartney on acoustic guitar backed by a string quartet. Initially, he wasn't too keen on the concept. "I said, 'Are you kidding? This is a rock group.' I hated that idea," McCartney told Granger. But after they worked on the track, McCartney and Martin were both pleased.

So was the public. The song was released as a single in the U.S. and soon shot to the top of the chart. Strangely, in the U.K., where it first appeared on the album "Help!," it wasn't released as a single until 1976. A singer named Matt Monro released a cover of the song there in 1965, which made the top 10. While Monro had other U.K. hits, perhaps his greatest musical contribution may have been starting a trend with "Yesterday." It went on to become the most covered song in history with more than 2,500 versions, according to The Guinness Book of World Records.

The inspiration for "Hey Jude," another one of McCartney's best-known Beatles hits, has been debated for years. The most logical explanation is that the song was inspired by John Lennon's first son, Julian, as his parents John and Cynthia were going through a divorce. Paul reportedly felt sympathy for the lad, but not enough to keep his name in the song's title. "Jules," short for Julian, was soon changed to "Jude" because it sounded better, much like "Scrambled Eggs" gave way to "Yesterday."

McCartney explained the song's origins in a 1973 interview with Paul Gambaccini for Rolling Stone and the BBC. "I happened to be driving out to see Cynthia Lennon," he said. "I think it was just after John and she had broken up, and I was quite mates with Julian. He's a nice kid, Julian. And I was going out in me car just vaguely singing this song, and it was like, 'Hey, Jules.' It was just this thing you know, 'Don't make it bad / Take a sad song...' And then I just thought a better name was Jude. A bit more country and western for me. In other words, it was just a name. It was like 'Hey, Luke' or Hey Max' or 'Hey Abe,' but Jude was better."

In a 1980 interview with David Sheff in Playboy, John Lennon confirmed McCartney's story, but then offered his own theory. "He was driving to see Julian to say hello," Lennon recalled. "He had been like an uncle. And he came up with 'Hey Jude.' But I always heard it as a song to me. Now I'm sounding like one of those fans reading into it. Think about it: Yoko had just come into the picture. He is saying, 'Hey Jude,' — 'Hey John.' Subconsciously, he was saying, go ahead, leave me. On a conscious level he didn't want me to go ahead."

Still others say McCartney was writing about himself, something that Lennon seemed to confirm in a 1968 Rolling Stone interview with Jonathan Cott. "Well, when Paul first sang 'Hey Jude' to me — or played me a little tape he made of it — I took it very personally. Ah, it's me! I said it's me. He says, no, it me."

There's little debate about the inspiration of "Let It Be." In fact, it's stated right in the song. It's McCartney's "mother Mary," although some read religion into the lyrics and have claimed it was Mary, the mother of Jesus. The song went on to top the charts, and while it was at the summit, it took on a different meaning when it was announced that the Beatles were breaking up on April 10, 1970.

Be sure to check out McCartney's recent surprise performance from Frank Sinatra High School in New York. It will be streamed Monday at 9 p.m. ET. McCartney's latest album, "New," is out on Tuesday.

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