eli5 How does razor blade dull on hairs when razor blades are made of steel and they are much higher on mohs scale?
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A very simple way of looking at the mohs scale is that it’s a scale of comparison. If steel leaves scratches on something but doesn’t recieve any scratches in return then the steel is significantly more scratch resistant (read: hard) than that something. The gimmick is that the steel was still scratched, but the ratio of damage between the steel and the something favored the steel significantly to the point that the steel *looked* undamaged to our eyes but the not-steel *looked* damaged. Small differences in hardness can result in extreme differences in practical scratch resistance though, so scratch testing typically pretty unambiguous.
But a razor edge on a blade is extremely fine (it has to be to cut things well) meaning that small damage done to that edge by softer things it encounters can add up pretty quickly.
The difference in hardness between the blade and the thing being cut does determine how quickly the blade wears down and loses its edge, but essentially any blade will eventually lose that nice sharp edge no matter what you’re cutting if you cut enough things. Hair is actually pretty hard for an organic material so it can cause damage to steel blades fairly quickly.
There are a lot more details to this of course, entire textbooks worth. The subtleties of this general principle are both highly elusive and very material-specific. It’s practically a whole field of mechanical engineering at this point and we’re still discovering details about how these interactions.
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