Answer:
I am always in front of you, but you will never see me. What am I?
It's like food but water kills it. What is it?
Kills the bad ones and the sad ones. Tightens to fit, so one size fits.
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.
My teeth are sharp, my back is straight, to cut things up it is my fate.
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