Lucy in the Sky with Diamonds is the song you're thinking of.

That would be “Lucy in the Sky with Diamonds.” John wrote most of it—the surreal lyrics, the melody, the whole vibe. Paul helped with the arrangement and added the bass line, but the core was pure Lennon.

The Lennon-McCartney credit was a blanket agreement they had from the start, so even when one wrote it alone, both names went on it. Same thing happened with “Yesterday” (Paul) and “Help!” (John). On Sgt. Pepper, “A Day in the