All Things Devours - Bilbo's Riddle, Op 208

Bilbo in the lair of Gollum answering this riddle:
This thing all things devours;
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats mountain down.
-JRR Tolkien
The answer to the riddle is, “Time”