The former One Direction singer and current late-night star drew names from a fish bowl, and the results were as goofy as one would expect. The duo traded off on vocals, accompanied by Horan's acoustic guitar and Fallon's plunky keyboard melodies.
Their first song, "Witness the Fitness," became an arena-rock power-ballad, with the singers shouting like professional wrestlers. "Quickly Clicky Pens" took on a jug band vibe, with Horan on the harmonica and Fallon blowing into the fish bowl. One improvisation sounded almost like a finished track: Fingerpicked folk-rock ballad "Stepping on Toes" chronicled a 15-year-old boy's experience at a high school dance, building to a series of booming vocal harmonies.