In order to pierce something, one would use high pressure. Pressure is defined as force per unit of area. A slice includes an additional force: friction. For example, saws have thin blades with jagged edges. The thin blade creates pressure and the jagged edges catch (due to friction) and tear the wood apart by cracking the wood fibers.

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Blades work in the same way but on a smaller level. Sharpening and polishing is done by creating tiny scratches on the metal. Concentration of the scratches at an angle on a thin piece of metal is what creates a sharp edge. The small scratches on the blade’s edge are like the jagged edges on the saw on a microscopic level. The combination of high pressure and the friction allows something to be sliced cleanly.

In contrast, a blunt knife doesn’t cut as well as a sharp knife, because it has less friction and greater surface area, due to rounding and smoothing.

A dull knife blade
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