Trainor throws a little attitude around (and plays a mean triangle) as she lets it be known, she's not a size 2, but that's okay since she can shake, shake it like she's supposed to.

Something Questlove and the Roots condone, letting her know she's got "all the right junk in all the right places." Fallon, who does seem stellar harmonizing, has so much fun singing the song he can barely contain himself by the end.

But his excitement is nowhere near the level of Trainor's when she learned that "All About That Bass" was the No. 1 selling digital song in the country in an interview with Mix 104.1 (a CBS Radio station).