Answer:
Three little letters, a paradox to some. The worse that it is, the better it becomes.
It is able to speak because it has a hard gone. You know what it is as soon as it has sung. What is it?
What comes once in a minute, twice in a moment, but never in a thousand years?
Die without me, never thank me. Walk right through me, never feel me. Always watching, never speaking. Always lurking, never seen.
Each morning I appear to lie at your feet, all day I follow no matter how fast you run. Yet I nearly perish in the midday sun.
You use it between your head and your toes, the more it works the thinner it grows.