David Letterman, who infamously told Bill O'Reilly in 2006 that "about 60 percent of what you say is crap," had his old bête noire back on his show on Tuesday night.

At the start of the show, Letterman couldn't help lashing into the Fox News host, saying "I wish I was smart enough to make him look ridiculous... He's going to come out here all puffed up."

From practically the moment O'Reilly sat down, the sparks flew...