Answer:
When there is fire in me then I am still cold. When I own your true love’s face then you will not see me. To all things I give no more than I am given. In time I may have all things, and yet I can keep nothing.
As round as an apple. As deep as a cup. All the king's horses can't pull it up.
I build up castles. I tear down mountains. I make some men blind. I help others to see.
In the evening I'm long, in the morning I'm small; When seen in a ballroom, I'm nothing at all.
I have a tongue, but do not talk. I follow close, but neither run nor walk. Coupled and hitched, but never married. You couldn't bear the load I've carried.
You use it between your head and your toes, the more it works the thinner it grows.