Many writers have the benefit of practiced foresight when building their worlds. The characters all belong to them and them alone. Things can run almost like clockwork. There’s a distinct order to fantasy novels, as an example, that simply is not present in a tabletop game.
However long you deliberate on the details of your world, eventually, as a DM, you will come up against the unexpected. Players can be just as creative even with their somewhat restricted control and understanding of the world you’ve built. Intelligent players will eventually point out flaws in the consistency of your unique ideas work. These are to be expected, enjoyed even. Putting your setting to the test will ultimately strengthen its depth of immersion for both you and your players.
When I first introduced that infinitely transformative substance to my players (see last article for more information: here), I was immediately hit with questions on how its transmutation interacted with physics as they knew it. What changes? What stays the same? Perhaps it was an uncommon approach for a player to make, that of a scientist’s experiments, but I welcomed it, even if I could not fully answer the questions asked.
Some DMs prefer players not to meddle in the minutiae of their worlds. It can be discouraging if someone picks apart how your cool and unique magic system works. This in itself can be an aspect of worldbuilding. Perhaps some god or being or even the magic itself does not enjoy being meddled with, and fights back against attempts to categorize and quantify it. These can become fun and interesting hooks for someone who is willing to experiment, to ask questions, to push the boundaries of what is known and unknown in your world; however, it’s important to not simply use these as a method of punishing curiosity. Players should be cautious, but not paranoid. Creativity should be rewarded but not overstep its bounds. This is why I find it more important to make rules for a setting before anything else.
Setting or making rules may sound restrictive. It certainly does not work for all worlds, for all stories; some are driven by the narrative tension more than by coherent laws. Even this can be accounted for, though: a lot of settings have beings that warp reality, that break rules, with or without consequences. Part of the fun of fantasy is that the forces of law and chaos can be personified, can be befriended, can be fought and vanquished. Remember that magic*, as a tool for games or story-building, inherently exists to break the rules of reality as we understand them. It’s all, as many players say, part of the fun.
(*Speaking of “magic” as a narrative tool, it isn’t always called such or even understood in the same way: sci-fi has advanced tech, horror fiction might have psychics or ESPers, fantastical monsters, the possibilities are only limited by your suspension of disbelief.)