The Melancholy of Haruhi-chan Suzumiya

That's not the face of a main character! (1x14)

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Haruhi and Kyon have a small battle involving balloons and blow-guns. However, when Mikuru wanders into the room, she is used as the unwitting victim of a balloon-decorating contest, with Itsuki as the judge. Meanwhile, Yuki finds some extra un-inflated balloons, and decides to take them home.