It's 'Twister', but not as you know it.
Inspired by an archive photo of Johnny Carson playing 'Twister' with Zsa Zsa Gabor from 50 years ago, Fallon invited the infamously awkward Stewart to a similar game, except this one involved downing (exceptionally large) Jell-O shots between movements.
Stewart, it turns out, is a pro and uneasily phased. Fallon, not so much...