8 Songs You Didn't Know Were Written by Katy Perry

This week we found out that Taylor Swift was the pen behind her ex-boyfriend Calvin Harris's latest hit, "This Is What You Came For" — but did you know that her frenemy Katy Perry has also written a number of songs for other artists? From Kelly Clarkson and Britney Spears to Swift's BFF Selena Gomez, Perry is no stranger to offering up tracks (and sometimes singing backup on them). Scroll through now for the surprising list, then check out the songs that you probably didn't know were written by Justin Timberlake and, of course, Sia.

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"I Do Not Hook Up" — Kelly Clarkson

Perry wrote this hit for Clarkson's All I Ever Wanted album in 2009.

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"Passenger" — Britney Spears

She cowrote this track with Sia for Spears's 2013 album, Britney Jean. It was originally intended for Perry's Prism album that same year but ended up being cut from the final release.

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"That’s More Like It" — Selena Gomez and The Scene

Yup, Perry gave this song to Taylor Swift's BFF Selena Gomez for her 2011 record, When the Sun Goes Down, with her band The Scene. Here's a leaked audio clip of Katy Perry's version.

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"Rock God" — Selena Gomez and The Scene

Perry provided the song "Rock God" for Gomez's 2010 album, A Year Without Rain, and also sang backup vocals. "I actually had to fight for that song," Gomez told MTV that year. "I wanted it so bad . . . I just loved her style."

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"Time's Up" — Ashley Tisdale

This track for Tisdale's 2009 album, Guilty Pleasure, was originally titled "Love Is a Train." Here is Perry's version of it.

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"Black Widow" — Iggy Azalea featuring Rita Ora

Who knew that Katy Perry was behind this smash hit? The song was passed on to Iggy Azalea for her 2014 album The New Classic after being cut from Perry's Prism.

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"Breakout" — Miley Cyrus

The song, which was released during Cyrus's Hannah Montana days, was originally meant for Perry's One of the Boys album in 2008.

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"Girl Next Door" — Jessie James

Perry wrote and sang background vocals on this song for country-pop singer Jessie James in 2009.