Jimmy Fallon's latest album features a protest song about the oil balls that were washing ashore after the BP oil spill called "Balls in Your Mouth," and after receiving a letter from a fan requesting that he perform it on the show, it's been on his mind.

So when Florence & The Machine's Florence Welch stopped by the show Wednesday night, it was as if the stars aligned.

The songstress joined the "Late Night" host in a stirring rendition of his very, very serious song.

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