Answer:
What goes inside boots and outside shoes?
The buyer doesn’t want it, the producer doesn’t need it and the user doesn’t know their using it.
Handy when you need to measure something or run a kingdom.
A father's child, a mother's child, yet no one's son
Big as a saucepan, deep as a cup, Even a river can't fill it up. What is it
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.