Originally Posted by Yora
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.