Taylor Swift's Red Tour rolled into Massachusetts over the weekend, and she had something special for the audience up her black-and-white striped sleeve: a surprise guest appearance from New England pop legend Carly Simon.

Swift introduced Simon to the crowd by saying, "Before I bring out my special guest, I have this question that I've always, always had. I think a lot of you have probably had this question too, always wondered what the answer is. Who is this song, 'You're So Vain' by Carly Simon, written about?" The duo then launched into a duet performance of Simon's 1972 hit, which topped the charts for three weeks and has been the subject of intense speculation ever since. You can watch the clip above.

Much like Swift in recent years, Simon made gossip-column headlines with her love life during the '70s, and the identity of the mystery man kissed off in 'You're So Vain' -- which includes the famous line "You probably think this song is about you / Don't you?" -- has long been one of rock's better-kept secrets. No word on whether Simon gave Swift the answer to her question backstage, but Swift did later tweet, "Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!! Unreal. I could not possibly look up to her more."

As many fans are no doubt already aware, the studio version of 'You're So Vain' featured harmony vocals from Mick Jagger, who shared the stage with Swift during a Rolling Stones show earlier this summer.

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