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Sharp blade undercuts water differently than a dull one. It is not easy to explain for me. Watch how the edge pushes the water wave forward and if/how the water moves up the blade. The sharper the...

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I can guess sharpness by how the razor cuts hair on my forearm. If it cuts hair above the skin without any effort (and it wants to cut into it when getting too close), I am there. If I have to...