Answer:
What do you throw out to use and take in when you're done?
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 is put on a table, cut, but never eaten?
What stays on the ground but never gets dirty?
In we go, out we go. All around and in a row. Always, always steady flow. When we'll stop, you'll never known. In we go, out we go.
You use it between your head and your toes, the more it works the thinner it grows.