They may be two of the most successful musicians in the industry right now but Taylor Swift and Selena Gomez are also two of the most grounded and down-to-earth celebrities in Hollywood.

The cute best friends teamed up to give Taylor's audience a little more than they bargained for this week during a concert for her Speak Now World Tour.

In the middle of her set, Taylor took a break to introduce Selena to the audience. Then, they performed Gomez's hit Who Says and the crowd absolutely loved it.

Selena isn't the first guest appearance on Swift's spectacular tour. She's also given audiences surprise appearances by Selena's beau Justin Bieber as well as country superstar Tim McGraw, rapper T.I., Darius Rucker, Nicki Minaj, Goo Goo Dolls, B.O.B, Usher and Jimmy Eat World.

Now that's how you put on a concert!

Of all of the guest appearances on Taylor's tour, who is your favorite?

Check out their performance below: