Answer:
You heard me before, yet you hear me again, then I die. Until you call me again.
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around
Only two backbones and thousands of ribs.
What can touch someone once and last them a life time?
When you stop and look, you can always see me. If you try to touch, you cannot feel me. I cannot move, but as you near me, I will move away from you.
I am rarely touched but often held. If you are smart you'll use me well. What am I?