Seeing as how this is the week between Christmas and New Year's Day, which is my vacation, I'm going to keep this one short. It may seem a bit obvious, but obviousness (if that's even a word) is dependent on what kind of science background you have. So today I'm going to explain why edged and pointed weapons work.
Pressure is one of the key components to causing damage to human tissue. Force is a component as well, but the area over which that force is applied can make the difference between a light massage and a stab wound. This can be seen from the formula for pressure.
P = F / A
where P is pressure, F is force, and A is area.
This makes sense from a practical position. If I lightly push my palm into someone's back, they probably won't feel pain because the force is distributed over the entire area of my palm. If I use the same force to push a tack into someone's back, then it's likely to break the skin and draw blood because the tack has a much smaller area. This is the reason that pointed weapons work. Weapons in general are just "force" multipliers in that they augment a similar action that is from an unarmed person. In this case, pointed and edged weapons "multiply" the force of a strike by dividing it by a smaller number. A slap may or may not cause much damage to a person, but a slash using a knife with the same speed and momentum probably would.
So, this idea of pressure is pervasive. It applies to punching and kicking and any other strikes. When punching, if you make contact with just two knuckles, it's going to have greater pressure on the opponent's body (and your hand, remember Newton's 3rd law of motion) than if you made contact with four knuckles.
Like I said, there's not really much that's surprising here, but if you stop to think about strikes and certain grappling (pain compliance) moves you can see a bunch of ways to apply the principle of pressure to your training. Now I'm off to fatten myself up a bit more so I can make more progress in my New Year's resolutions :-).