Answer:
What gets sharper the more you use it?
In buckles or lace, I help set the pace. The farther you go, the thinner I grow. What am I?
I am a kind of coat that can only be put on when wet. What am I?
I am of several different kinds, but the one you pick doesn't do its job. What am I?
I have a straight back and sharp teeth to cut objects.
I repeat only the last word you say. The more I repeat, the softer I got. I cannot be seen but can be heard. What am I?