Answer:
who do you think you know but sometimes you don't
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
I am black when I am clean and white when I am dirty?
What is it which builds things up? Lays mountains low? Dries up lakes, and makes things grow? Cares not a whim about your passing? And is like few other things, because it is everlasting?
What can be swallowed, But can also swallow you?
You use it between your head and your toes, the more it works the thinner it grows.