Answer:
This thing all things devours: Birds, beasts, trees, flowers; Gnaws iron, bites steel; Grinds hard stones to meal; Slays king, ruins town, And beats high mountains down.
Left to right Iām quite bright. Right to left I might bite.
Come up and let us go. Go down and here we stay.
Give it food and it will live, give it water and it will die.
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around.
Iām grown from darkness but shine with a pale light. Very round I am and always a lady's delight.