What Am I Riddles

Question: Swallowing one is fine.
Swallowing two together is fine.
But Swallow them separately and all shall fall.

What are they?

Question: All things I devour: Birds, beasts, trees, flowers.
I gnaw iron, bite steel.
I grind hard stones to meal.

What am I?

Question: I plow and plow, but never sow.

What am I?

Question: I can be sun,
I can be sand,
and I can be bird.

What am i?

Question: I sit high glowing and watch the world go by, without me everything would seem dull and dangerous. What am i?