For the third night of their recent residency on 'Late Show With David Letterman,' the Foo Fighters invited swamp-rock legend Tony Joe White to join them for a performance of White's signature 1968 song 'Polk Salad Annie.'

White may not be a household name for many rock fans, but he's been active for nearly 50 years, releasing a long string of highly regarded albums and writing songs covered by Elvis Presley and Tina Turner, among others. He's clearly got a fan in Letterman, who strolled onstage after the song ended and told everyone in the room, "I’ll tell you something, and this is just between us: If I was this guy, you could all kiss my ass."

Of course, as evidenced by their lengthy stay on the show, Letterman's also a big fan of the Foo Fighters -- and in the web-exclusive clip below, he shares part of the reason why, relating the story of how after his son Harry was born in 2003, "I recognized I was older than most fathers, and so did my son. And so I said I have to find something I can do with my son and we can do it together." That "something" ended up being a ski trip that he and Harry shared in the mountains of Montana, and their instructor used 'Miracle,' a song from the Foos' 'In Your Honor' LP, as the music bed for footage of the two skiing together.

"This is the second song of theirs that will always have great, great meaning for me for the rest of my life," noted Letterman before the performance, which you can watch below -- and then watch the Foo Fighters performing with Heart here, as well as their duet with Zac Brown on Black Sabbath's 'War Pigs' right here.

Foo Fighters Perform 'Miracle'