Why is popcorn verboten when you have braces?

It seems pretty soft to me. Is is because of the unpopped kernels, or is it because of something else?
Update: @Katty: yea, I'm approaching my 1-year mark, and I've figured the same thing. I was just wondering, since I went to the movies this past weekend and got popcorn, but I made sure to spit out any unpopped kernels that ended up in my mouth.
Update 2: @MagnusMoss: I wasn't quite sure how to verbalize what you said, which is why I wrote it as "something else". I was hoping that wasn't the reason.
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