Answer:
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.
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.
It is able to speak because it has a hard gone. You know what it is as soon as it has sung. What is it?
I bubble and laugh and spit water in your face. I am no lady, and I don't wear lace.
Who is he that runs without a leg. And his house on his back?
What never gets any wetter no matter how hard it rains?