Answer:
I am the greatest teacher out of them all, but alas in the end I kill my pupils.
What has a mouth but can't chew?
Different lights do make me strange, thus into different sizes I will change.
I weaken all men for hours each day. I show you strange visions while you are away. I take you by night, by day take you back. None suffer to have me, but do from my lack.
My teeth are sharp, my back is straight, to cut things up it is my fate.
When is it bad luck to see a black cat?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?