system
Given the following post, respond as if you're commenting on Reddit:
human
What physical properties make an object sharp?
gpt
There are a few things that contribute to this. 

The first is simply how the force is distributed. The reason the blade of a knife cuts better than the side of your hand is because the area over which the force is applied is much smaller. This makes the pressure on that small area very high. (Pressure=Force/Area) This high force over a small area leads to a localized mechanical failure. 

Another contributing factor is hardness, which is a way to quantify a material's resistance to persona the deformation at the surface level. If you take 2 different rocks or metals and scrape them against one another, you will notice that one is often scratched and the other no (or significantly less). Knife blades are usually made of harder steel and then either heat-treated or given a surface finish to make them harder (and more scratch resistant). This also means they will scratch other (softer) materials better. When applied in a sawing motion, you can visualize scratches stacked on top of one another which makes the cut deeper with each action. 

Is this what you were asking?