Lady Gaga knows how to spell out "Bad Romance," but last night, she was an advocate for "Ordinary Love."

Gaga, who beautifully covered John Lennon's "Imagine" last month, continued to honor the rock-elite last night (July 26) when U2 invited her on stage at Madison Square Garden for a performance of their ode to the late Nelson Mandela. The rockers have been on tour since May to support their Songs of Innocence album.

"Her Ladyness, Her Gaganess," Bono introduces the pop act to the eager crowd in the clip above. Gaga, dressed in a black bikini (and those "Bad Romance" heels!), soon assumes a seat behind the piano for the sullen tune, and though the U2 front man is first to open his mouth to sing, his tone is quickly eclipsed by Gaga's big, bellowing notes.

"Your heart is on my sleeve / Did you put it there with a magic marker? / For years I would believe / That the world couldn't wash it away," the singers belt in the performance to uproarious applause. If a project with Tony Bennett was successful, we can only imagine how an album featuring these two would perform.

"Thanks for letting me sit in last night guys @U2, thanks for ripping with me in the garden. That was special, plus I love Irishmen," Gaga tweeted this morning (July 27). We're sure the Emerald Isle feels likewise...

Check out the performance, and tell us if you dug The Gaga/U2 collision!