While running a game you need to have a quick answer whenever a player asks "why?"

Preferably the answer is something that makes sense and is convincing within the context of the game world and the game rules, and not just "Because I say so."

Transcript

Yoda: Double, double, toil and trouble! Dung of swamp slug! Brain of brain spider!
Yoda: Ready, your dinner is! Ahahaha!
Luke: {sotto voce} More like toilet trouble.
R2-D2: I'm going to keep an eye on Luke. Up periscope. {peeks inside the hut}
GM: Roll.
R2-D2: Um, why?
GM: Because you were just eaten by a dragonsnake and spat out into an unpadded tree.
{beat}
R2-D2: Fair enough. <roll>
[SFX]:
GM: Okay, I can't read Quenya. What does it say?
{beat}
R2-D2: <sigh> It says "your periscope is ".