Answer:
More precious than gold, but cannot be bought, Can never be sold, only earned if it’s sought, If it is broken it can still be mended, At birth it can’t start nor by death is it ended.
Its title means book.
My teeth are sharp, my back is straight, to cut things up it is my fate.
I am mother and father, but never birth or nurse. I'm rarely still, but I never wander.
You can see nothing else, When you look in my face, I will look you in the eye, And I will never lie.
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?