Quote Originally Posted by Yora View Post
You can always just say "Your device does not work". Mixing charcoal, sulfur, and salpeter does not make gunpowder. You also have to know the right ratios and preper it in the right way, or you get something that just burns without a bang. And good luck with a combustion engine. Let me see how you get that crude oil to become an aerosol.
More like, "how does your character know that?"

The problem starts when your players reply: "Well, ok, I want to run so-and-so experiment" (which is an IC reasonable experiment to try) and the GM figures that the answer going to end up with floating volcanoes or something crazy.

Which, I hasten to add, happens rather often when a magic system isn't built from the ground up.

Case study:
Spoiler
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As a troublesome prospective player to a game that hadn't worked out its magic system yet, I asked a few questions regarding the magic system (questions were encouraged so I asked).

As it turns out, defining "object" and "stationary" was a bit more difficult than it seemed at first glance. Some weirdness resulted with my thought experiments for mages in sea combat destroying the ship when he cast a defensive shield that slowed incoming projectiles (the ship he was on counted as moving under the definition of stationary).