Answer:
A hole leading in, a hole leading out, we connect to a cavern that is slimy all throughout. What are we?
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 is often returned, but never borrowed/
In the land of the green glass door there are riddles but no answers, sheets but no blankets, and books but no words. Name something found in the land of the green glass door.
Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not
You use it between your head and your toes, the more it works the thinner it grows.