What's a sooner?

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Why: In "The One Where Emma Cries":

Monica: Tulsa, Oklahoma?!
Chandler: The Sooner State! Whatever that is!

Answer: A sneaky lil cheater:

In 1899, the US government opened up 2 million acres of land to homesteaders. To claim a lot of land, prospective settlers had to participate in a land run. They lined up and waited for the blast of a shotgun to signal the beginning of the run, at which point they would race eagerly to claim a homestead. A "Sooner" was someone who snuck past the territory markers ahead of the shotgun to get an early start.

Source: Ask Yahoo!

The More You Know: Did you ever see Far and Away? It was one of the 3 movies that Tom Cruise did with Nicole Kidman as part of their 10-year marriage contract. Anyway, the movie is kind of crappy, but Tom Cruise's Irish accent is hilarious and they do this in it - during the Oklahoma Land Run of 1893. Jicydk.