Answer:
A house of wood in a hidden place. Built without nails or glue. High above the earthen ground. It holds pale gems of blue.
Many have heard it, but nobody has ever seen it. It will not speak until spoken to. What is it?
I can be written, I can be spoken, I can be exposed, I can be broken.
What does everyone know how to open but not how to close?
What does a WW1 Trench, A Submarine and a Popular Phone App all have in common?
You use it between your head and your toes, the more it works the thinner it grows.