While promoting his new film The End of the Tour, Segel agreed to a game of "Word Sneak," which consisted of Fallon and Segel attempting to slip random words they received on index cards casually into their conversation.
Predictably, laughter ensued.
Fallon, a veteran of the game, seamlessly dropped the word "frankfurter" into conversation when Segel mentioned he came on the show early in Fallon's talk show career.
"Oh, you came on when I was just like a dancing frankfurter running around NBC," Fallon says.
Later, Segel proved he's a quick study when he deconstructed the name Gene Shalit – featured on his card – to the words "jeans" and "shallot."
When Fallon told Segel he's in good shape and asked if he has changed his diet, Segel answered as though a lightbulb just went off in his head.
"It's funny you should ask if I'm changing my diet because I have an actual answer to this," Segel replies. "I don't want to wear like big, baggy like overalls or something, so I've been eating food to help me get into the stuff I like. Well, if I want to wear some pants, maybe I'll eat some onions: Gene(s), Shalit."
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The joke sent Fallon and the audience into hysterics, with Fallon baiting Segel even further when he mentioned that Gene Shalit works in the building.
"Oh, that's a man also?" Segel replies.
Watch the entire amusing exchange above.