Answer:
The root tops the trunk on this backward thing, that grows in the winter and dies in the spring.
They have not flesh, nor feathers, nor scales, nor bone. Yet they have fingers and thumbs of their own. What are they?
What begins and has no end? What is the ending of all that begins?
What goes in, but never through? I've got some and so do you. We might lend one, or even two, but only if our trust is true.
I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.
You use it between your head and your toes, the more it works the thinner it grows.