Human beings cannot quite accurately be described as bags of water ... although that is often used as a joking comment in describing the percentage of water which makes up our composition. The actual percentage varies ... from 55% to 78%, depending on how hydrated you are at any given moment, as well as how old you are, whether you’re male or female, the climate you live in and so on. My point is that there is a difference between hitting you in the chest with a club, as opposed to hitting a free standing column of water appearing to be the same size and shape as you ... but it isn’t what you think.
Let us imagine such a column of water, and let us further imagine that the water is somehow able to keep its shape – magically, if you like. In any event, there is no hard membrane within the water, nor outside it. And finally, let us suppose this water column is YOU.
Swinging the club through water, I will encounter some resistance – depending on the relative thickness of the water. As the thickness is the same as you are, I will probably be able to complete a swing completely through the pure water version of you. The resistance will probably make me grunt, and depending on the thickness of the club I might have to bend my elbows some at the end of my swing.
If, on the other hand, I try to drive the club straight down through your pure water head, chances are the club won’t get as far as your torso. Something as narrow as a baseball bat might.
But let’s say I don’t use a club, I use a sword. Now obviously, the sword is going to cut through you, and quite easily, no matter which direction I swing. There’s a terrific benefit to the sword having a sharp edge.
Obviously, I don’t think any of this is news to the gentle reader – but I want to put you in the right frame of mind.
Let’s go back to you being you again, composed of blood, bone, sinew, gray matter, what have you. Your body is better designed to withstand the beating of clubs and the cutting of swords – whatever pain they might cause – as you are, to some degree, armor plated. If I were to swing a sword against your naked body, I would do damage – I might even kill you. But the sword would not pass willy nilly right through your body, as though you were made of water. Of course, you don’t re-assemble as easily as water, but your complex body provides you with other gifts. Being able to hold a sword, for example.
But there is another aspect to being hit with clubs and swords which I haven’t mentioned. Your body moves.
Let’s say I hit you in the chest with a club, just as I did when you were composed of water. This time, as the club hits you, your ribs flex inward, retaining your integrity, and your whole upper body ripples, redistributing the blow. Every joint down to your fingertips will loosen and separate, as in effect your body momentarily ‘liquefies.’ You also fall down, or at least you fall back ... further distributing the momentum of the blow to your environment.
I’d like to bring up an interesting point – your mass as a flesh and blood creature is actually less that it would be if you were composed of pure water. This is not merely because you have air in your body; the various composite parts that your body is fashioned of – blood, bone, sinew and so on – are as a whole less dense than water. Bone, for example, is only 81% as dense as water. Your overall specific density is about 0.9 grams per cubic centimetre, compared water, which has a density of 1.0.
The human body – any animal’s body, really – would make poor building material. The old saw about a body dropped into a cornerstone of a stone building under construction would soon liquefy and weaken the whole structure. Cracks would form and the building would collapse.
Except for your ability to redistribute force when struck by it, you’re not a very resilient material for making fortifications.
Obviously, stone works much better. If I make an equivalent of you made with stone, suddenly even the sword doesn’t work very well. The club is a dead loss. Both weapons lack mass, and the club lacks density as well ... spruce has a specific density of only 0.45 – half as much as you. Even the hardest woods are only as dense as 0.93.
Granite, on the other hand, has a density of 2.7. That means that a granite statue the size and shape as you weighs three times as much. In terms of resistance against force, that’s considerable.
This has been a long way around the barn, but I want to make a point, and I want us to be on the same page. When we compute the amount of damage done by a sword or an arrow against flesh, there is a point where the amount of force is so negligible that no damage is done. If I gently slap a sword against my palm, the sword does not penetrate my skin and I experience no noticeable injury. If I were to do it a thousand times, I would still fail to cause a single hit point of damage – though my hand might be a little stiff, with an ordinary rush of adrenaline it would probably continue to function normally. The same is true for a club swung against the chest of a stone statue. Though you stand there all day, hammering away with the club, at some point you might succeed in overcoming the club’s resistance (breaking it), but the stone is going to be just fine.
Where computing the damage done against a fortification, it helps to keep this sort of thing in mind. A ballista bolt will have more than a superficial effect on you ... but even at point blank range, it isn’t likely to do much damage to a stone wall. To determine such damage, a threshold of force must be taken into account. If your weapon only applies force that's less than that threshold, no effect.
I feel this is worth noting on a post all of its own because it applies to other aspects of D&D as well – often overlooked. How much damage, really, can a quarterstaff do against a dragon covered with scales? Or a club against an oliphant? Accepted, the weapon might hit, but does it break the skin? Does it imaginably bruise the skin? Would it make a difference if an 18/00 strength was behind it ... given that the club’s mass is still the club’s mass, no matter how hard it is swung. Isn’t it more likely that the 18/00 strength would break the club against the dragon’s hide, before it would break the dragon’s hide?
I’m nowhere close to proposing rules to cover these things. I’m struggling just now with applying some sort of threshold in the amount of damage done to castles and ships ... and I am at a loss. I can’t find much data – that I can comprehend – on the effects of resistance against force, along the lines I’ve just described.
Sometimes, I’m just not smart enough.