Answer:
My teeth are sharp, my back is straight, to cut things up it is my fate.
Large as a mountain, small as a pea, Endlessly swimming in a water-less sea.
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.
What does everyone know how to open but not how to close?
When the horse strokes the cat, the wood begins to sing.
Said to have fifty shades
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?