Taylor Swift sure knows about some loud reactions.  She gets tons of screams from her fans night after night, but even Taylor was amazed at the reaction she got with her special guest that joined her on stage and L.A.'s Staples Center.  In the middle of her Speak Now Tour set, Taylor welcomed fellow sensation with the teenage crowd, Justin Bieber to sing his hit "Baby."

Taylor Tweeted: You think you've heard LOUD screaming in your life.. Then @justinbieber comes out and does a surprise song during your show. Woah. Unreal.

Take a look at the surprise appearance.