Katy Perry's Demo vs. Britney's 'Passenger' & 9 Other Demo vs. Final Version Showdowns

Mike Windle/Getty Images for iHeartMedia
Britney Spears performs during 102.7 KIIS FM's Jingle Ball 2016 presented by Capital One at Staples Center on Dec. 2, 2016 in Los Angeles.

Every hit you blast in the car and on the beach starts somewhere. Sometimes it's with a demo singer in a studio, who lays down a guide track that helps one of your favorite stars spin it into gold. But sometimes it's a fellow star, who helped write the song and has their own demo laying around that has its own special magic. 

Case in point: this week a demo co-written by Katy Perry and Sia of the Diplo-produced "Passenger" leaked online. The track eventually ended up on Britney Spears' 2013 Britney Jean album, after the crew decided it was better suited to Britney. Happens all the time.

Check out nine other demos and final versions of some of your favorite songs below.

Ne-Yo's demo of Beyoncé's "Irreplaceable" is more country-ish than you might expect.

Sia's "Diamonds" is pretty, pretty close to the finished Rihanna smash.

Former 90s pop star Cathy Dennis co-wrote Spears' "Toxic" and her demo is pretty close to the final.

Sia's original version of "Perfume" had a bit more raw drama than the final Britney version.

Avril Lavigne helped write one of Kelly Clarkson's signature hits, "Breakaway," which is so close to the final you wonder why she didn't keep it for herself.

Skylar Grey is an amazing singer and songwriter, who put an extra layer of sad on her demo for the Rihanna/Eminem smash "Love the Way You Lie."

Ed Sheeran helped write the global smash "Cold Water" with Diplo, Nick Kibler, Mø, Justin Bieber, Benny Blanco and others, and it's a toss-up whether his vocals would have made it as big as Justin's.

"Party in the U.S.A." co-writer Jessie J definitely put some stank on Miley Cyrus' signature song.

Turnabout is fair play, as Britney did a grimy demo of "Telephone," written for her by Lady Gaga, who eventually took it back and recorded it with Bey.