Here's why:
* Hardness is a material's resistance to scratching or indentation. When you scratch one material with another, you're testing how easily one material can penetrate the surface of the other.
* The harder material will scratch the softer material. This is because the harder material has a stronger atomic structure that resists deformation.
For example, a diamond is incredibly hard and can scratch almost any other material. A fingernail is relatively soft and can be scratched by a piece of glass.
