Funny vs. not-funny: Seinfeld gives an example

Not funny: "There's no important calls. You know how you know the calls not important? They're calling you."

Funny: "There's no important calls. First of all, the people you know, they're not important people."

Logically, they're (almost) the same joke. The second version gets a laugh. The first doesn't. The two versions direct your attention differently. Seinfeld didn't know that until he tried them on stage.