"Paul needed to jam and that's what we did. And on that particular day he said he had another song. It was called 'Yellow Submarine.' He played it for me and said, 'There's just one line I haven't got.' So I said, 'Let's go into the other room.' We did and I came out with 'Sky of blue and sea of green' - my only contribution to the Beatles' lyrics. So Paul and I had a productive little relationship there for a while."