Waterbomb base
The waterbomb base is a traditional starting shape in which a square collapses into a triangle with two flaps on each side.
Crease both diagonals, turn the paper over, and crease both midlines. Then push the left and right edges inward along the horizontal midline while bringing the top half down; the paper folds itself into a triangle with its point at the top. It takes its name from the traditional inflatable waterbomb, or paper balloon, that is folded from it.
It is the inside-out twin of the preliminary base, so the usual mistake is placing the diagonal and midline creases on the wrong sides. If you get a square instead of a triangle, reverse which set of creases you made first. On this site it forms the Butterfly’s wings and the Jumping Frog’s front legs.
