Filling water balloons is a simple task -- keeping them in one piece is where the challenge lies. For example, the balloons can pop as you fill them, or they can pop when you store them, leaving you with a sad little pile of balloon pieces rather than an arsenal of plump water weapons. To minimize popping and maximize splatting, fill your balloons carefully, handle them with respect and store them safely.
Get the Water Right
If you overfill a water balloon, it can pop before you even tie it, but don't be tempted to underfill it -- this makes it less likely to make a satisfying explosion. Finding the sweet spot may take some trial and error, so have a backup supply of balloons on hand. Typically, you should fill the balloon until it starts to bulge and stretch, stopping a couple of inches from the top. If you can't tie it easily, it's too full and you should let out some water.
Once you tie a balloon, it's primed to explode; handle it as little as possible, ideally with wet hands so your skin doesn't catch on the surface of the balloon.
Store Water Balloons Carefully
Storing the balloons so they don't pop as just as important as properly filling them. Containers such as coolers, pails or even trash cans are ideal; however, don't just pile the balloons into the container. As you pile them up, the weight of the balloons on top can pop the ones below.
Instead, fill the container with water so the balloons can float rather than put pressure on each other. If you prefer not to use water to fill your container, try a plastic trashbag: This allows the balloons to spread out and may relieve the pressure. It's also a good idea not to store filled water balloons in sunlight, as if they get too hot, they'll start to explode.