Answer:
I move without wings, Between silken string, I leave as you find, My substance behind.
What covers its face with its hands, speaks no language, yet most known what it's saying?
It doesn't live within a house, nor does it live without. Most will use it when they come in, and again when they go out.
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 never was but always will be. No one ever saw me but everyone knows I exist. I give people the motivation to better themselves every day. What am I?
You use it between your head and your toes, the more it works the thinner it grows.