| void | | | | | cluster | | like an asteroid belt it contains a few large objects (like vesta) many small asteroids and billions of tiny pieces of rock and dust, not touching, with finite gaps between.
Or like this image of the contents of a volcanic cloud. |  | | tree | | like a real tree but with no branches touching. Also can be thought of as many hills on fewer larger hills recursively, no loops/self connections, like the shape of this explosion if you were to think of the cloud part as solid.
So it's an unlooping hierarchy of fused solids. | | | sponge | | a network of thick connections amongst a larger network of thinner connections recursively. Would let water in and through. Basically nested loops.
This particular image shows a special kind of sponge, its complement is the same shape just rotated 180 degrees.
More generally the complement of a sponge form is a different shape sponge. |
| | shell | | like branching planes, a shell is a covering around a tree form. You can also think of it as recursively many indents inside fewer larger indents like this cave. | | | foam | | solid containing a cluster of gaps, so like amber or glass with bubbles in it. | | | solid |  | a dense volume of matter, usually considered as rigid, or at least to have a stiffness and finite stretch. | |
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