I've got chills and they're multiplying.  And I'm losing control over this version of "You're the One That I Want" from Grease.  Trust me!  You have never heard that song in this way before.  Check out how X Factor duo Alex and Sierra performed the John Travolta/Olivia Newton-John classic.

With this performance, Alex and Sierra earned a spot in Simon Cowell's final four.  He is mentoring the Groups category this season and has a powerful one-two punch with Alex and Sierra.  They're unique, inventive and a blast to watch.

The X Factor airs Wednesdays and Thursdays on Fox.