"I cast Create Water in his lungs!" "I cast Fabricate to make a nail out of his hemoglobin!" "I cast Plant Growth on the sunflower seeds he ate earlier!"
These are all garbage, tired takes, used extensively by Tiktok creators wanting to get views and the uncreative nerds that watch them. And I say this as someone who *absolutely* loves unorthodox spell and item use. Yeah, there's nothing in the rules that say you *can't* do it, but I feel that, more often than not, it ruins the narrative and tone of the scene, not to mention the mechanics of whatever encounter you're in. So, how do you stop it from happening?
The most obvious and easy answer is communicate with players and say that it's off limits, or go so far as to say "If I let you do it, then we establish that it's a thing that can be done and enemies are going to do it to you." That stops most, but some players love to push things to see how far it can go, and then it turns into a game of Unethical Wizard Rocket Tag where martial characters hide behind a rock while spellcasters invent new and horrible uses for Message that somehow explode brains.
Thus, I would like to present a homebrew I like to use. No new rules, but rather putting rules in context that give DMs a reason to say no to using spells intended for objects on people apart from "because I said so": The biological concept of homeostasis.
For those that didn't pay attention in high school biology or forgot: homeostasis is the state of a living organism where all of its regulatory systems and internal gooshy bits are balanced with each other. A living body is never doing nothing, but all the stuff it *does* do balance out to a place where all those systems can keep working as best they can. Further, if that equilibrium is disrupted, the body will do what it can to fix it and return to that state of neutral homeostasis, be in healing a wound, fighting infection, regulating a mental state, all that stuff. Kind of like a gyro; if it's not moving at all, you can turn it and rotate it or whatever, but if it's spinning, suddenly it becomes much harder to manipulate, and always seems to want to go back to how it was.
So what does this have to do with Unethical Wizard Rocket Tag? Apply the same concept to magic and a soul. When you use a spell on a rock, there's nothing resisting the spell other than physics. It doesn't take much effort to start to do something. On the other hand, a soul has a concept of "self", knowing what it is physically and magically, and that self includes the area immediately inside and around its corporeal form, which is why anything that targets a creature usually also affects its equipment in a similar fashion, like Enlarge/Reduce or Polymorph. It's that continuing sense of self that pushes back and resists a spell.
If you think of a spell slot as a set amount of energy to put into a spell, and then compare spells of a similar level against each other, then you'll notice a trend: spells that don't target creatures tend to have much larger overall effects than those that do. That's because, in order to affect a creature, you have to put a portion of a spell slot's energy into overcoming the creature's soul homeostasis. Too little and you don't break through, too much and the spell doesn't have enough energy to do what it actually needs to do. Objects don't have a soul, so all that spell slot energy can go into the effect. For sake of the argument, constructs like golems and whatnot have an artifical homeostasis brought about by the animating magic, so it applies to them, too.
Importantly, the sense of self extends to the area inside a creature, too, as well as, for the most part, equipment. So Create Water, while it could theoretically spawn water in someone's lungs, the soul considers the space in its lungs to be a part of itself and resists, and since that spell was not designed with creatures in mind, resisting it is trivial. Same with Plant Growth; the inside of the stomach is still, according to the soul, a part of itself, even if it is topographically outside the body.
So yeah, a neat, in-universe explanation to head off degenerate spell usage logic. Again, it would be easier to just communicate with players, but even the best of communication can break down from time to time, and so this is here to be a worldbuilding net.
