Answer:
Three little letters, a paradox to some. The worse that it is, the better it becomes.
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
Take one out and scratch my head, I am now black but once was red.
I am a fire's best friend. When fat, my body fills with wind. When pushed to thin, through my nose I blow. Then you can watch the embers glow.
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
You use it between your head and your toes, the more it works the thinner it grows.