Answer:
I run through hills. I veer around mountains. I leap over rivers and crawl through the forests. Step out your door to find me. What am I?
I am longer than a decade and shorter than a millennium
What can be as big as an elephant but weighs nothing?
Sometimes I am white, sometimes I am dark. Most people love me. What am I?
I shrink smaller every time I take a bath. What am I?
No matter how little or how much you use me, you change me every month.