Answer:
You use me between your head and your toes the more I work the thinner I grow.
I tremble at each breath of air and yet can heaviest burdens bear.
A piece of paper that takes most people 3 or 4 years to get.
I spend the day at the window go to the table for meals and hide at night.
I have no voice but I can teach you all there is to know. I have spines and hinges but I am not a door. Once I've told you all, I cannot tell you more. What am I?
The answer I give is yes, but what I mean is no. What was the question?