In D&D, certain types of energy are associated with certain types of classical elements, via creature properties, cleric domains, items, etc. Fire corresponds of course to Fire. Water usually gets Cold. Earth gets stuck with Acid, and usually Air gets Electrical. What I'm seeking to understand is why might this be, from an IC standpoint. OoC, I'm sure it's mainly mechanic based, but it boils down to a certain representation of some system in the actual world. There's no real wrong or right way I guess to interpret these things, but some valuable insight might come out of discussing it.
Fire and Fire: I think this is self explanatory, but let me not write it off too quickly. Often in D&D "steam" effects will do "fire" damage too, like a Steam Mephit's breath, Jet of Steam, and Fire spells used underwater. So while we often think of Fire damage as always being burny, the common denominator here is actually
heat. So assumedly, things that have resistance to Fire ingame most likely also have resistance to general heat sources. Again, maybe that's obvious to some--no one expects a Fire Genasi to pass out from heat stroke--but it's certainly interesting to consider.
Water and Cold: This is where it gets interesting. Water is not in itself an "energy." Most Water effects in D&D, where the water has a non-harmful temperature, it actually translates into physical damage. I.E.
The Decanter of Endless Water. The NWN spell,
Drown, does bludgeoning damage. The Monster ability Drown does magic damage if I remember, but I would bet that's simply to make the damage irresistible in vanilla NWN.
However, when we want to associate something with Water in NWN, we also give it something to do with Cold damage. Potions of Cold Resistance, Water Domain gains resistance to Cold in their prayer, etc. In the end, it seems necessary. D&D isn't pokemon, where you can squirt water at stuff and it hurts for no reason. But, short of basically any metal, water is a decent heat sink. there's plenty of it and it has a high specific heat which is why we use it in cooling units. So while water itself is not directly associated with "Cold energy" (the engineer in me still won't allow me to say that without quotes even though this is a theoretical universe) it certainly is the most readily available vehicle to transfer that energy. So we get things like Ray of Cold, Cone of Cold, which would directly be associated with the Plane of Ice, being associated with plain old Water. So, somewhere along the line, beings of Cold and pointed at water and were like "yo, that shit. That's shit's alright by us. Let's make it our thing." *Shrug* It's all I got.
Earth and Acid: This seems tricky, but I think it actually makes the most sense of any, short of Fire v. Fire. When we talk about Earth in D&D we often combine what is both physical earth (dirt, minerals, rocks) with what grows from the earth (plants, fungi, etc). This is how we get Chauntea thrown in with deities like Grumbar by having Earth in their domains. So, somewhere between leafy, organic, plant stuff and hard, inorganic rock stuff, we have a connection, and that's Acid. So we look at what exactly Acid is.
Acid, by the D&D definition, is essentially a chemical burn of some sort. Corrosion, when it comes to organic things or inorganic things. While it's called "acid" we can assume we have bases and oxidizers grouped in with it, any sort of chemical reaction that does structural harm. When something like flesh undergoes one of these sort of reactions, we get organic compounds being changed into other sorts of compounds. I'm not bold enough to say molecules and blah blah work like they do in the real world, but it's safe to say there is still some sort of alchemical transition that happens in D&D and FR too. Something goes from one thing to another.
My thoughts on this is that that same "acid" energy is also associated with things like decomposition and digestion in the game universe. In fact, there are a few creatures in D&D (like the Tarresque) where characters take acid damage after being swallowed whole. In that sense, Acid is an energy associated with the changing of states. Odd, given that elemental earth is often associated with unchangingness, but there you go. Plants take in and digest minerals from the earth (an "acid" reaction in D&D), plants eventually die and decompose back into these base minerals(another "acid" reaction). So again, while elemental Earth is not an energy just like Water and Air are not, it is thoroughly involved in the workings of that energy. In this case, Earth is actually the thing being transported by the energy, instead of transporting it like Water v. Cold. In any case, we might be able to interpret the "acid" effects involved with either Earth creatures or given by beings like Chauntae to be some sort of natural decomposition.
Air and Electricity: This one I'm still shaky on. I am aligned with the thinking of the Avatar universe where Electricity is actually "refined" Fire. I know that technically thermal energy and electromagnetic energy are two different things, but we're talking about kung fu magic here, so meh. The reason I disagreed however was that it always seemed to me like Sonic was the better choice here.
Sonic energy, which is often Sound, can be transferred through the air. In terms of the game universe, it's treated as an "elemental" form of energy, like fire, cold, acid, and electricity (the Endure Elements series and Elemental Resistance feats show us this). When I think about being hurt or physically impacted by air, I naturally think about having an air horn blared in my ear or feeling the shockwave from some sort of explosion. It seems like it would be a kinetic form of energy associated with it, like water. I suppose one can say that Air is the vehicle for lightning, given that electricity travels through the air molecules. But air under most conditions sucks at conducting electricity. Saline water is a better conductor (and it's interesting that the
Bronze Dragon has the Water subtype and a lightning breath weapon possibly for that reason). Then again, water is also a much, much better conductor of sound than air.
Maybe Air was just the unwanted step-child in the whole ordering of energies thing and got whatever was left over. On a mythological level, a lot of sky deities are associated with lightning, so maybe it's that. It still seems a bit weird to me that gods related to Air and flying creatures should be so fond of lightning, something that should scare the bejesus of anything out there that flies in the sky. But then again, Cold freezing a water elemental seems pretty scary too, or an earth elemental being eaten away by acid. I guess fire could always be consumed by a bigger fire as well. Maybe the Elements are all invested with the energies that could harm them most. I'm interested to hear anyone else's thoughts on this sort of thing.