Answer:
In marble halls as white as milk, lined with a skin as soft as silk. Within a fountain crystal-clear. A golden apple doth appear. No doors there are to this stronghold, yet thieves break in and steal the gold.
I can run but not walk. Wherever I go, thought follows close behind.
No matter how little or how much you use me, you change me every month.
I turn around once. What is out will not get in. I turn around again. What is in will not get out.
Different lights do make me strange, thus into different sizes I will change
What covers its face with its hands, speaks no language, yet most known what it's saying?