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Jar of flies is an activated item. Added in Afterbirth, it starts empty. Each time an enemy is killed, a friendly blue attack fly is added to the Jar and the counter in the top left corner goes up, to a maximum of 20. Once activated, the flies in the Jar are released, forming a ring of flies around Isaac. These blue attack flies will seek out enemies and cause damage. If these flies kill enemies, the counter does not go up, however kills by other familiars will increase the count.