Answer:
I am free for the taking. Through all of your life, though given but once at birth. I am less than nothing in weight, but will fell the strongest of you if held.
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.
What can burn the eyes, sting the mouth, yet be consumed?
The root tops the trunk on this backward thing, that grows in the winter and dies in the spring.
My voice is tender, my waist is slender and I'm often invited to play. Yet wherever I go, I must take my bow or else I have nothing to say.
I build up castles. I tear down mountains. I make some men blind. I help others to see.