"You're So Vain" Taylor Swift-Carly Simon Duet Blows Fans Away-- Simon Revealed Who Song Was Really About [VIDEO]

"You're So Vain" was sung on stage by Taylor Swift and guest Carly Simon. Watch the video below.

Carly Simon's "You're So Vain" is the original breakup anthem...and Taylor Swift has followed in her footsteps with her own breakup ballads (after all, she's never, never, getting back together with a certain un-named someone) Thus, the two bringing it back by singing on stage together makes sense.

Before the show, Swift was giddy in anticipation, saying"Once we get out there and we have 55,000 people in a sing-along it's just going to be like karaoke night in the stadium. Every girl out there will be thinking of someone when we're singing," Swift said.

Swift and Carly Simon sang the 1973 hit together at a concert in Massachusetts for Swift's Red Tour.

Unsurprisingly, Taylor was wearing red shorts and striped top for the performance in Foxbrough, Mass.

"Before I bring out my special guest, I have this question that I've always, always had," Swift told the crowd. "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?"

Carly Simon came out on stage at the Gillette Stadium to sing the song and fans roared. The crowd lost it...then found themselves as Simon and Swift sang the vengeful classic.

She didn't answer the question-but apparently, Carly told Taylor and swore her to secrecy.

"Ladies and gentleman, give it up for one of your own, Carly Simon!" Swift shouted when they were done.

A backstage interview with the two will be aired on Good Morning America. Swift called the performance "one of the most special nights of the tour."

Swift also Tweeted about the experience, revealing that she's a big fan on Twitter.

"Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!!" she wrote. "Unreal. I could not possibly look up to her more."

She said during an interview, "When I heard 'You're So Vain' I just thought that is the best song that has ever been written," she added. "That is the most direct way anyone has ever addressed a breakup. It's amazing."

Simon enjoyed herself, too.

"It was just so wonderful that I just want to do it again," Simon said of the performance. "[Taylor] was so great, and she really led me through my own song, and she was just so loving of the whole thing, and we loved each other and we moved together in a part. It was dramatic, it was great."

"It's a song that really sort of developed that style of mine," Simon told press. "A lot of it was not knowing that I developing that style because lot of things happen by accident."

Watch video here!