Answer:
This only turns over once you have traveled very far. What is it?
who do you think you know but sometimes you don't
Some try to hide, some try to cheat. But time will show, we always will meet. Try as you might, to guess my name. I promise you'll know, when you I do claim.
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
What can be measured but not seen?
You use it between your head and your toes, the more it works the thinner it grows.