The Rolling Stones' most enduring classics have been part of our collective soundtrack for so long that it can be easy to forget that they all had to start with an idea. In a new interview with Harper's Bazaar, guitarist Keith Richards reflected on the events that helped shape the writing and recording of three of the band's biggest hits and longest-lasting fan favorites.

Arguing that there are "only two kinds of songs ... the beautifully smooth ones that are plainly obvious when they come to you, and the ones that give you a hard time," Richards looked back on "Gimme Shelter," from 1969's Let It Bleed; "Sweet Black Angel," from 1972's Exile on Main St.; and "Beast of Burden," from 1978's Some Girls — all of which were directly or indirectly inspired by factors that wouldn't have been immediately apparent to listeners.

Of "Gimme Shelter," for example, Richards said that the song's intense lyrics — which, years later, seem to sum up a period of armed conflict and overall social turmoil — weren't initially any part of the inspiration behind the music. Instead, he was inspired to grab his guitar after literally watching people trying to get out of the rain.

"I had been sitting by the window of my friend Robert Fraser’s apartment on Mount Street in London with an acoustic guitar when suddenly the sky went completely black and an incredible monsoon came down. It was just people running about looking for shelter — that was the germ of the idea," said Richards. "We went further into it until it became, you know, rape and murder are ‘just a shot away.’"

Today, "Gimme Shelter" is arguably best remembered for the vocal contributions of singer Merry Clayton, whose late-night summons to the studio was memorably recalled in the 20 Feet From Stardom documentary. As Richards remembers it now, Clayton's involvement was another gradual addition to the track — and one that represented a creative departure for the band.

"I can’t think of a time I ever started a song off saying, ‘This is going to be a duet,’" said Richards. "But somewhere in the process of making the record it suddenly became obvious that we needed a female voice. Mick and I both looked at each other and said, ‘Man, we need a bitch in this!’ So [producer] Jack Nitzsche called up the singer Merry Clayton, and she was at the studio within an hour, and we cut it. Just like that."