This is a great GM line. "Mmm. I wonder why that is?"
Use it whenever a player questions something in the game as being unrealistic or weird. Whether or not you actually have a pre-planned explanation for it, or if it was just an oversight or something that you didn't bother researching to make it realistic.
Instead of arguing with your players, you acknowledge that they're right to think something is strange, and you establish that there must be some plausible explanation for it. If only they could be clever enough to figure it out.
And if you have any players anything like Jim, they can help you figure out what the explanation is. And then you run with it like a thief.
Commentary by memnarch (who has not seen the movie)
Or not. No rescuing here. I guess we're not done with the movie writer confusion as it's time for a cave adventure!
I'd actually guess that BB and Poe are similar in density. BB might be made of metal, but it's not like droids are solid objects and he wasn't that difficult to move in Episode VII. Plus, a large sphere isn't going to have a lot of different moving surfaces for the ground to be moved by. Humanoid shapes, on the other hand, would have that "advantage" which would be how metal-man Threepio gets to already be standing on the floor while Poe is still picking himself up.
I do like the set-up the last comic had, even if Pete's roleplaying rule doesn't seem like it'd save him if he tries pushing it too hard. Goopy quantum soup aside.... I guess this could be another wacky Dark Side slime pit? Rey had that fall into one last Episode, and since that was on an island, it was full of slime and water. Here on probably Tatooine, we've got sand and gravel everywhere, so this is just gonna be rocks, rocks, and more rocks. Up until we find another funhouse mirror and get to see everyone have fun trying to figure that one out.
Transcript
GM: BB-8 emerges from a gravel ceiling and falls to the floor of a tunnel.
[SFX]: clatter... clatter...
[SFX]: Thump!
GM: Poe’s already on the floor behind you.
BB-8: How did you fall through that faster?
Poe: Maybe I’m more dense.
BB-8: You said it, not me.
C-3PO: Where are the others?
GM: A few pebbles drop and then Rey emerges from the ceiling.
[SFX]: clatter... clatter... clatter...
Poe: This isn’t right. The gravel should just collapse.
GM: Mmm. I wonder why that is?
Rey: The liquefied space-time I’m carrying. Producing spatial anomalies.
[SFX]: Thud!
GM: Chewbacca drops to the tunnel floor.
Chewbacca: How fortunate your quantum goop happens to explain everything just when we need it.
Rey: Pete’s Rules of Roleplaying #71: Plant narrative excuses ahead of time so the GM feels guilty about killing you.
GM: That’s adorable.
GM: Make a Dex roll at -4 to see if you’ve broken a leg.