Answer:
No matter how little or how much you use me, you change me every month.
I am the key that is the hardest to turn
The more you add the less it weighs. What is it?
I go up when the rain comes down
Only one color, but not one size, stuck at the bottom, yet easily flies. Present in sun, but not in rain, doing no harm, and feeling no pain. What am I?
It flows out of the soil, It burns you if it boils, And holds us in its coils, More valuable than gold, As black as it is old.