They weren’t in cars getting coffee, but on Friday night Jerry Seinfeld and Norm Macdonald had a funny — and somewhat puzzling — exchange on the latter’s show.

Though best known for his “What’s the deal with…” style of observational humor, Seinfeld is no stranger to more heady fare. Case in point: what he describes as “the greatest Jew joke that I’ve ever heard,” though it comes with a catch: Macdonald and other non-Jews might not get it. “That’s the joke, is that you won’t get it.”

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“The setup of the joke is that it’s a Jew joke,” Seinfeld explains before actually telling it: “Two gentile businessman meet on the street. One of them says, ‘How’s business?’ The other one says, ‘Great!'” Macdonald is nonplussed, while his co-host Adam Egert (who,...