Answer:
I open wide and tight I shut, Sharp am I and paper-cut fingers too, so do take care, I'm good and bad, so best beware.
You heard me before, yet you hear me again, then I die. Until you call me again.
Through its wounds, water does run. It once held many but now has none. What is it?
I am nothing really at all, Yet I am easily foundΝΎ Ignore me at your own peril, and you might end up crowned!
The higher I climb, the hotter I engage. I cannot escape from my crystal cage.
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?