They both tried to get their songwriting chops on the same page in this segment of The Tonight Show's Instant Song Challenge, for which fans submitted song-title ideas, and Horan and Fallon then had to whip up a jam on the spot with little more than their wit, an acoustic guitar, and a keyboard.
Hilarity ensued — how could it not when the first prompt is "Witness for Fitness," and they turn that into a super-metal ode to workouts? — as did a race to see whose face could get as red as possible in the shortest amount of time. Turns out that the topic of exercise bodes really well for some serious bellowing and minor-chord action, as Horan and Fallon definitely work as a joke of a metal duo.
Or, maybe it's not such a joke to the "Slow Hands" crooner, as he was pretty stoked about it: "We need to release that!"
The rest of the segment is basically the equivalent of two dudes who rocked out at the karaoke spot for approximately 12 beers too long, but Fallon's ear and ability to harmonize is impressive — even if Horan can't make it through a line without cracking up.