Answer:
Iron on a mammal. Mammal on your feet.
You use it between your head and your toes, the more it works the thinner it grows.
Glittering points that downward thrust. Sparkling spears that never rust.
Metal or bone I may be, many teeth I have and always bared. Yet my bite harms no one. And ladies delight in my touch.
I am never quite what I appear to be. Straight-forward I seem, but it's only skin deep. For mystery most often lies beneath my simple speech. Sharpen your wits, open your eyes, look beyond my exteriors, read me backwards, forwards, upside down. Think and answer the question...
What language does a billboard speak?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?