Rock goes pop in this week's Mashup Monday as Florida's Yellowcard puts their guitars (and violin!) to California girl Katy Perry's hit "E.T."

Photos: Yellowcard's Mashup Behind-The-Scenes

Yellowcard at Billboard

"There's a hook that goes through the whole song that's a really staccato synth part and having Sean [Mackin] playing violin was a really cool way to have that part in an acoustic setting," singer Ryan Key says of Yellowcard's unique take on the song, which peaked at No. 1 on the Dance/Club play chart this spring. "We used the guitar and the violin to really emulate it and do an octave higher and an octave lower, which is how they do it on the record."

While the two artists could not be more different musically -- an alt-rock band from Florida with punk leanings and a pop superstar from Cali who started out in Christian music -- Yellowcard and Perry have crossed paths before. Make sure you watch the whole video above to hear Key tell the story of how Perry once sat in onstage with Yellowcard several years ago, before her string of huge hits.

"I'm a big fan of Dr. Luke and Max Martin who work with her on a lot of songs," Key continues. "This song, I think, the melodies are just so rad, really original."

Yellowcard, meanwhile, has been very busy this year. Latest album "When You're Through Thinking, Say Yes" rocketed to No. 5 on the Rock Albums chart last month and the band is currently on a huge world tour that takes it through Canada, South America, and Europe before they head back to the states for a full U.S. tour this fall.

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MORE VIDEO FROM YELLOWCARD'S BILLBOARD VISIT: