Answer:
I know a word of letters three. Add two, and fewer there will be!
What makes a loud noise when changing its jacket, becomes larger but weighs less?
Hold these between your knees if you want to set a Cuban beat.
What I am filled, I can point the way. When I am empty. Nothing moves me. I have two skins. One without and one within.
I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?