Origami Water Balloon
The origami water balloon (also called water bomb) is an easy to make, working toy.
Start with a square of paper.
Then make mountain and valley folds as if you were making a square base.
The difference is that the mountain and valley folds are reversed when compared to the square base.
Origami Water Balloon
Now start to collapse the pre-folded piece of paper. 
You will end up with the folded triangle shown below. This piece occurs so often in origami that it has its own name, the balloon
base (or water bomb base). 
Now fold each of the four corners upwards as shown in the
next three photos:
Now fold the four horizontal corners into the center as shown:
Use your finger to expand the little pocket as shown below. You are going to tuck the flap into the pocket.
However, the flap must
first be folded in two to make it small enough for the pocket.
This sequence shows the flap folded, then tucked in.
Now fold and tuck in the other three flaps.
Finally expand the balloon as shown below. First unfold the balloon partially, then blow gently into the hole at one end.
At our school, we actually filled these with water for some
recess hi-jinks...
