How to Fill a Water Balloon. Water balloons keep kids occupied while draining their youthful energy, making for peaceful summer nights. Balloons, and the water it takes to fill them, cost pennies-on-the-dollar-worth of fun. Become the coolest house in the neighborhood: Fill up water balloons for backyard games. Supervise children playing with water balloons so that none are thrown at people or animals from a high perch, such as a several-story building, or with an intent to hurt.