Answer:
I tremble at each breath of air and yet can heaviest burdens bear.
What is made of wood, but can't be sawed?
I'm not really more than holes tied to more holes; I'm strong as good steel, though not as stiff as a pole.
I reach for the sky but clutch to the ground. Sometimes I leave but I am always around.
You find me behind the stars; or in a sixth seventh or third. It takes something round a computer and me to make pie. I am bigger than anything you can think of.
What is brown, big and red all over?