More like, "how does your character know that?"
Originally Posted by Yora
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.
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).