Answer:
What do you fill with empty hands?
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.
I turn around once. What is out will not get in. I turn around again. What is in will not get out.
I move without wings, Between silken string, I leave as you find, My substance behind.
It's in your hand though you can not feel it. Only you and time can reveal it.
The warmer I am the fresher I am.