He looked at Simon and then back at the piano keys. She became flushed.
"I harmonised with him as it became a chorus, an improvisation that was later searched for and never found among the multitracks at AIR Studios," Simon wrote.
Never found - until, perhaps, now.
A new duet, never before played in public, was recently discovered on a tape owned by Matt Lee, a Rolling Stones collector in London, the Associated Press reported. The song goes by the title Fragile, the AP said, citing Rolling Stones fan websites.
The lyrics recalled by Simon match Lee's recording, though the two seem to croon the word "change," not "cry."
As the closing notes ring out, a female voice utters a gasp: "Good song," Simon affirmed, according to the AP. Lee said he sent a copy of the song to Rolling Stone magazine, which told him it would transmit it to Simon.
In an interview with the magazine in 2016, Simon stoked speculation about the long-lost duet.
She sang the line she had put in her book, asking, "Does that sound like any Stones song to you?"
The interviewer suggested Fool to Cry, a brooding 1976 ballad that finds Jagger seeking solace from his daughter and then his lover. Both rebuff him, deeming him "a fool to cry."
"Maybe, maybe," Simon said. "We had this little back and forth at the piano for about an hour."
But then Paul and Linda McCartney arrived, she said, and the spontaneous collaboration was over. She said she didn't know what happened to the recording - or whether any trace of it remained. Richard Perry, a producer, had been looking for a tape for years, she said, speculating that it had to be lurking somewhere at Warner Bros.
Although Lee declined to tell the AP where the tape came from, he did offer a review of the duet on Twitter, writing, "It's super great!"
According to the AP, a small part of the song, including the "funny, funny, funny" lyrics that Simon remembered, can briefly be heard in a documentary about the 1972 Rolling Stones tour. The documentary has never been publicly released.
Lee made his finding two years after No One Loves You More Than Me, recorded by the Stones in 1964 and apparently sold at an auction, was located. The band finished a European tour in July and has said a new album is in the works.
Publicists for the two singers didn't immediately return requests for comment about Fragile.
Simon told Rolling Stone in 2016 that she had last seen the lead Stones vocalist at an after-party following an awards show, when saxophonist Bobby Keys picked her up and set her down in front of Jagger. "I was very uncomfortable," she said. Before that, she had tried to catch his eye at a concert of his in Boston. That was 12 years ago.
In her memoir, she wrote that "for Mick Jagger, all women, including me, were his, by divine right."
But he wanted something particular from her, she believed. Something as fragile as a Shakespearean romance.
"And if Mick could have his way, it would be Romeo and Juliet tragic," she wrote. "We couldn't have each other."
