Answer:
Until I am measured I am not known, Yet how you miss me when I have flown.
What do people want the least on their hands?
What stays on the ground but never gets dirty?
What I am filled, I can point the way. When I am empty. Nothing moves me. I have two skins. One without and one within.
A house full, a yard full, a chimney full, no one can get a spoonful.
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.