Plants are wonderful things. They can take raw elements out of the ground and turn them into something useful. For example, an apple tree is a tool to turn atoms from the ground into apples.

Suppose humans could expand on this idea. Suppose we could send a ‘seed’ (be it biological or mechanical) to a planet, moon, or asteroid, and it used raw elements to builds a factory. This machine, in turn, used raw elements to build a base, probes, ships, or whatever we programmed it to build.

How could such a seed work? It could send out ‘root's’. These roots find the raw elements and send them back to the seed where it begins to grow a factory. As the factory grows, more roots are sent out further from the seed. The factory will use these same roots, and grow new ones, to supply it with building material.