John Mayer Writes a Song for Taylor Swift

Taylor Swift's amazing year just got better - John Mayer has written a song for her.

The Love Story singer is a big fan of the rocker but never dreamed she'd get to sing one of his songs.

She says, "I was so excited when he Twitter-messaged me. He was like, 'Hey, I just wrote this song this morning and I can only hear you singing on it with me.' I was like, 'Yes!'"