How did popcorn become a popular snack at movie theaters?

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Dear Yahoo!:
How did popcorn become a popular snack at movie theaters?

RoxannNorth Braddock, Pennsylvania

Dear Roxann:
No trip to the movies is complete without an overpriced tub of popcorn covered in goo. Good thing, 'cause popcorn sales are responsible for a substantial portion of theater profits.
To learn how the tradition began, we first consulted the Encyclopedia Popcornica. According to this industry-supported site, the salty snack was very popular "from the 1890s until the Great Depression." Even during the Depression, popcorn remained "one of the few luxuries down-and-out families could afford."
Kidz World writes that popcorn first became available at movie theaters way back in 1912. They don't explain why, but Buzzle.com came through with many kernels of truth (none of which remained unpopped).
Apparently back in the old days, popcorn vendors would "set up shop" outside theaters. The theater managers didn't like this, thinking it was a distraction. But moviegoers disagreed, frequently ducking out to buy popcorn and then ducking back in to see the movie.
As Buzzle further notes, "it wasn't long until the theatre owners realized they could set up their own popcorn popper." That's exactly what they did, and popcorn's been associated with movies ever since. Kind of like Cracker Jacks and baseball games or gruel and orphanages.