In order for knives to be useful, they need to be sharp. If I am sharpening one myself, how do I know when I am done?
What would be a simple easy test of whether a knife is sharp enough?
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Try to slice a piece of paper. A good sharp knife makes a clean cut. A dull knife makes either a ragged cut, or worse, just pushes the paper to the side. I like this article: https://www.fieldandstream.com/articles/hunting/2014/08/paper-cut-testing-blade-sharpness#page-4 and YouTube is full of videos of people showing off their knives via this test.
The best method in my opinion is one that must be learned, and involves moving your thumb across the blade, perpendicular to the blade not down it! It's very difficult to describe the feeling but if you do it on enough dull blades and sharp blades you will begin to be able to tell the difference.
One other thing I will do if im sharpening a knife for a friend or coworker to display how sharp it is is shave a couple hairs off the back of my hand. Lick your thumb, moisten the patch on the back of your hand and shave as you would with a straight razor. (You may want to wipe the knife clean after this.) Sounds weird but i do it because your average joe will know its extrememly sharp if you can shave with it and be careful with it.
One other test could be to attempt to very thinly slice a tomato, with and exceptionally sharp knife you should be able to get nearly paper thin slices with a light sawing motion.
Generally I categorize sharpness into 3 levels:
The tomato test - being able to slice through a tomato skin. Not use the pointy end to break the skin and then slice but directly slice the skin all the way through. This is sharp enough for cooking.
The paper test - being able to slice thin phonebook paper or magazine paper or newsprint. For general carving knives I find some tearing after the knife has started a cut to be acceptable. If I really want sharp then it should be able to cut paper with zero tearing.
The shaving test - you should be able to shave with it. This is literally razor sharp (otherwise known as "scary sharp"). I've only been able to do this on some knives and I wouldn't want all my knives to be razors anyway.
Cheap, disposable sponges. Unlike paper, they can be tested on while wet, which is a big advantage when sharpening with waterstones. Don't use the scrubby side (some people like using it as a deburring aid, though)
Something dull will not be able to cut into a sponge. Something sharp will easily slice into it. Something d... sharp can push-cut into the corner of a sponge. Something b...y damn sharp can be laid on a sponge and pushed in (a razorblade easily will!).