Answer:
You use it between your head and your toes, the more it works the thinner it grows.
Not born, but from a Mother's body drawn. I hang until half of me is gone. I sleep in a cave until I grow old. Then valued for my hardened gold.
Only one color, but not one size. Stuck at the bottom, yet easily flies. Present in sun, but not in rain. Doing no harm, and feeling no pain.
A harvest sown and reaped on the same day in an unplowed field. Which increases without growing, remains whole though it is eaten within and without. Is useless and yet the staple of nations.
Thousands lay up gold within this house, But no man made it. Spears past counting guard this house, But no man wards it.
I march before armies a thousand salute me. My fall can bring victory but no one would shoot me. The wind is my lover one-legged am I. Name me and see me at home in the sky.