"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 mountain down"

A: Time

"What has roots as nobody sees,
Is taller than trees,
Up, up it goes,
And yet never grows?"

A: A Mountain

"A box without hinges, key, or lid,
Yet golden treasure inside is hid."

A: An Egg

"It cannot be seen, cannot be felt,
Cannot be heard, cannot be smelt.
It lies behind stars and under hills,
And empty holes it fills.
It comes first and follows after,
Ends life, kills laughter.

A: Dark