Bells is a weapon used by genjutsu users. They can be used to make illusions of the user to drive the enemy nuts or cause hallucinations when the enemy hears the sound of the bells. This technique has only been used by Kin, during her fight with Shikamaru during the preliminary rounds of the Chunin Exam.
Also, when used for throwing needles, bells can be used to disguise others. For example when a shinobi throws a first set of needles with bells attached to them, and then the second set will be thrown with only half of the needles having bells on them so the shinobi on the receiving end will only dodge the ones with the bells on and not notice the ones without the bells.
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