Nothing

Thirty spokes make a wheel
They meet in the middle: it is the hole that makes the wheel go.

A clay pot has a shape too,
But there is nothing inside. And that's the useful part.

This room has doors and windows.
It's useful because of the holes in it.

Something valuable can be touched.
The useful part is nothing.

[See Chapter 11 of the Tao Te Ching by Lao Tsu]