Answer:
What is the thing which, once poured out, cannot be gathered again?
I have many feathers to help me fly. I have a body and head, but I'm not alive. It is your strength which determines how far I go. You can hold me in your hand, but I'm never thrown.
My children are near and far. No matter that I know where they are. The gift I give them make their day. But if I were gone they would wander away.
What lies in a tunnel of darkness. That can only attack when pulled back?
A dagger thrust at my own heart, dictates the way I'm swayed. Left I stand, and right I yield, to the twisting of the blade.
You use it between your head and your toes, the more it works the thinner it grows.