According to legend, Led Zeppelin wrote "The Rain Song" after George Harrison told them that writing some ballads might be a good idea. (Apparently, he hadn't spent much time with their catalog.) They took him up on the challenge, writing one of their most mellow and lush songs. Taking a page out of the Genesis playbook, they even put a mellotron on the track. "I don't even know what kind of music this is," Rick Rubin told Rolling Stone in 2011. "It defies classification. There's such tasteful, beautiful detail in the guitar and a triumphant feel when the drums come in – it's sad and moody and strong, all at the same time. I could listen to this song all day. That would be a good day."