Riddle of the Day

[Gizmokid2005]

Mountains will crumble and temples will fall,
and no man can survive its endless call.
What is it?

ANSWER (Congrats Mathster): Time.

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NEXT RIDDLE:

A time when they’re green, a time when they’re brown,
But both of these times, cause me to frown.
But just in between, for a very short while,
They’re perfect and yellow, and cause me to smile!
What am I talking about here?