Answer:
My life can be measured in hours, I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.
What is often returned, but never borrowed/
There are millions of me. I am very small but when moving fast I am deadly.
People like this usually live in rural areas, unless they are lucky enough to strike oil and move to Beverly Hills.
Different lights do make me strange, thus into different sizes I will change.
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?