This was Gollum's final riddle from The Hobbit:
"This thing all things devours;
Bird, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slay king, ruins town,
and beats a mountain down."
I make ash, but I’m not a bonfire.
I can expel rocks, but I’m not a slingshot.
I can be a mountain, but I’m not in the Himalayas.
I have a crater, but I’m not the Moon.
I erupt, but I don't have a bad temper.
What am I?
It is energy and motion, it can fly across the ocean.
It can listen and even speak, it can see and even seek.
It eats neither fruit nor gruel, it eats energy and fuel.
Better say: It doesn't eat; It is transformed into heat.
What is it?