Ratmachines.com

Thirty spokes unite in one hub;
It is precisely where there is nothing, that we find the usefulness of the wheel.
We fire clay and make vessels;
It is precisely where there's no substance, that we find the usefulness of clay pots.
We chisel out doors and windows;
It is precisely in these empty spaces, that we find the usefulness of the room.
Therefore, we regard having something as beneficial;
But having nothing as useful.