How come knives can cut easily through fabric and food but not rock?

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How come knives can cut easily through fabric and food but not rock?

In: Physics
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A knife is a tool that takes the force applies to it and focuses it along a very narrow edge. This concentrates the force in one place, and helps it to break the bonds holding the material you are cutting together. Those bonds have different strengths in different materials, making them correspondingly easier or harder to cut. If the material you are trying to cut is even stronger than the material that the knife is made of, then most of the force will wind up going towards deforming and wearing away the edge of the blade instead of cutting through the material.