Taylor Swift Full Fan Girl at Justin Timberlake Show

12/15/2014 7:29 AM PST

Taylor Swift -- Full Fan Girl at Justin Timberlake Concert


It's unclear how Taylor Swift has time to learn the lyrics to EVERY song -- in addition to her own -- but she does ... and she unleashed her super fan lip-sync skills on Justin Timberlake this weekend.

Swift hit up JT's show in Brooklyn Sunday night to cap off her birthday weekend. T-Swizzle went into flailing arm mode, and not only sung along, but acted out every song too. 

It's great stuff ... unless you're the person sitting behind Taylor.