1x14 - The Melancholy of Haruhi-chan Suzumiya

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.