Answer:
You use it between your head and your toes, the more it works the thinner it grows. What is it?
It can be clear like water, but it burns like fire.
What freezes after it is heated up?
What is it that given one, you'll have either two or none?
The more I appear The less you see. So riddle me this: What could I be?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?