Between 1962 and 1965, the Beatles had obeyed the unwritten rules for writing pop singles: they should have love as their subject; be just two minutes long; and be easily reproducible in concert. Now, gradually, they were enjoying seeing how many of these rules could be broken while still retaining the immediacy and excitement of chart music. Paperback Writer was their first non love single; Eleanor Rigby and Rain were the first of their songs to chart but not be played in concert; and Yellow Submarine was their first singalong song.

Paul chose only short words because he wanted it to be a song which would be picked up quickly and sung by children. While writing it, he visited Donovan at his flat in Maida Vale. "We were in the habit of just dropping in on each other," remembers Donovan. "I was just waiting for the release of my album Sunshine Superman and so we played each other our latest songs. One of the songs Paul played me was about a yellow submarine, but he said he was missing a line or two. He asked me if I'd like to make a contribution. I left the room for a bit and came back with 'Sky of blue and sea of green, In our yellow submarine'. It wasn't an earth shattering creation but Paul liked it enough to use it on the eventual recording."

Released as the flip side of Eleanor Rigby in August 1966, the same month that Revolver came out, the rumour quickly spread that the yellow submarine was a veiled reference to drugs. In New York, Nembutal capsules started to be known as "yellow submarines". Paul denied the allegations and said that the only submarine he knew that you could eat was a sugary sweet he'd come across in Greece while on holiday. These had to be dropped in water and were known as "submarines". "I knew Yellow Submarine would get connotations," said Paul, "but it really was a children's song."