AceShowbiz -

Taylor Swift and Zac Efron delivered a fun duet during their visit to "The Ellen DeGeneres Show" recently. They strummed their guitars with the "New Year's Eve" actor being assisted by the "Mean" singer. After playing one chord, he's encouraged by her, "That sounds so good!" In response, Ellen DeGeneres jokingly said, "Only a girlfriend would react that quickly like that."

The two stars, who serve as voice cast on upcoming animated movie "The Lorax", were rumored to be dating lately. They, however, denied the romance report and told DeGeneres that she taught him how to play guitar. "He's really good at it, like one lesson," she praised him. "I taught him how to play 'Pumped Up Kicks'."

Instead of sticking to the original lyrics of the Foster the People song, they altered the lines. "Ellen works a long day/ giving weird interviews in a slick cool sweater," Efron sings. "And it gets kind of weird/ Calling us out like we're boyfriend and girlfriend/ But it's not 'cuz we're just sitting here."

Swift jumped in with her own revised verses, "I've been on this show 8 times now/ And I still remember Ellen hiding in the bathroom with a hidden camera/ She scared me so bad I fell and I could've died/ She always asks me who I'm dating every time I'm on the show/ And I don't even know why."