Katy Perry’s spirited attitude and powerful voice made her one of the defining pop stars of the early 2010s. Her songwriting acumen helped, too: She cowrote some of her biggest hits, like the up-from-doldrums anthem “Roar” and the winking “Hot n Cold.” Her fingerprints are also all over songs she penned for other artists, like Kelly Clarkson’s feisty “I Do Not Hook Up” and Ashley Tisdale’s bittersweet “Time’s Up.”