Here goes. The first part is to answer the riddle itself, the second part may be more difficult.
"This thing all things devours;
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays kings, ruins towns;
And beats high mountain down."
What is it?
And, what book does it come from?
[This message has been edited by El Lurch (edited 06-03-2001).]