A fluid mixing system includes a flexible bag having a first end, an opposing second end, and an interior surface bounding a compartment; a retainer coupled to the second end of the flexible bag, the retainer having a retention cavity formed thereon that communicates with the compartment of the flexible bag; and an elongated member having a first end rotatably coupled to the first end of the flexible bag and an opposing second end freely disposed within the retention cavity of the retainer so that the second end of the elongated member can rotate within the retention cavity relative to the retainer.