Round things, like bubbles, float...just like the imagined gravity-free beings. Or roll around. They can be anywhere; they are fixed nowhere. And, like a round egg (have you seen spider eggs?), a round (or spherical) object is often a self-contained "world" that has everything it that it needs to sustain itself without having access to anything beyond it. As long as the contents are "sealed into" that round or spherical container, they can exist on their own. Once the bubble breaks, however, they enter our world.... Round artworks are not meant to break. They are meant to be seen as a vision of another world that's not in our world at all and that is never going to be. Something beyond our reach. This is one reason that religious subjects are especially appropriate subjects for round artworks.