When you blow soap bubbles, do you ever produce cubes or pyramids? Why are all the bubbles spherical? Bubbles seek to form the lowest energy configuration, the one which encloses a given volume within the smallest surface area. A sphere, with its infinite number of "sides", is that shape:
Likewise, when we dip rigid structures built out of Zometool components into a soap solution, the resulting "bubbles" may not be what we expect. As with free standing bubbles, these systems seek to form the lowest energy configuration.