Taylor Swift's tour stop Saturday night in Foxborough, Massachusetts, was highlighted by a special guest. The "Red" singer, who's known for her own bitter ballads, brought out Carly Simon to cheers throughout Gillette Stadium, before the pair proceeded to sing the latter's 1972 smash "You're So Vain."

"I think a lot of you have probably had this question too, always wondered what the answer is," Swift said to the audience before Simon's entrance. "Who is this song, 'You're So Vain' by Carly Simon, written about?"

Watch fan footage of the performance below: