Imagine a story that takes place entirely in a bar, with friends drinking and talking. At one point in the story an empty beer glass flies off a table (by itself) and crashes through a window. Other than telling us the glass does that, the story never mentions the incident again, and no characters pay any attention to it.
So now imagine I call the story's author and say the beer glass scene doesn't work for me, because I want to know more about why the glass flew and why no one seemed to notice or care.
AUTHOR: I didn't explain the glass because I wanted to show, not tell, that this fictional world looks exactly like the real one but is different in a couple of ways.
ME: Okay, you did that. But why didn't anybody react?
AUTHOR: Because I wanted to show, not tell, that in this world, people are so used to seeing beer glasses fly around by themselves that they don't even notice it half the time.
ME: Would they react the same way if a vodka bottle flew around?
AUTHOR: Oh no. [Explains why beer glasses are the only objects that fly around.]
ME: That explanation makes sense. Here's the problem. I had to call you in order to learn it, and learning it makes me appreciate your story and your story's world a lot more than I did before. Your story has to give readers, in its text, the same appreciation you just gave me over the phone.
AUTHOR: But I don't want to interrupt my story for a three-page information dump.
ME: I don't want that either. I want you to acknowledge that readers are going to wonder why the beer glass moves on its own, and address that question in the story.
AUTHOR: There's no way to do that without the story being artificial. No character in this world would bother mentioning a flying beer glass. It would be like a character in the real world going outside in the rain and saying "Hey, look at this water falling from the sky," then explaining what weather is and what water is.
ME: What you described is the worst way to handle the problem. On the other hand, if your rain story's audience lived in a world where water had never fallen from the sky, you would have to address rain for that audience the same way I'm asking you to address the beer glass.
At this point, if the author were still reluctant to explain the flying glass, I'd ask the author to reconsider having one in the story at all. If this fictional world has flying beer glasses then it probably has other strange departures from the real world too, yet those aren't in the story. So why put in something that will just end up annoying the reader?
Here are two approaches to solving the problem.
1) A character says, "I hate when that happens. At least this one wasn't full." The next character says, "Do you complain about everything that happens every day? What are you going to yell at next, the sun for setting every night?" Or maybe, "That's what you get for not chaining it to the table," and have the character dangle the unused chain at whoever complained.
You could argue this conversion is a bit artificial. I'd argue all stories are, and must be, artificial, but that's another post. If you don't like the dialogue solution, try this one:
2) [Narration:] Most bars had switched to paper or Styrofoam cups to avoid the problem of flying beer glasses, but the Redrum Bar's owner thought the danger added atmosphere. He couldn't have been too wrong; the bar was packed every night.
You could argue the second approach is no good because it tells instead of shows, but I'm going to argue against that idea in my next post, next week. The point is, if readers are obviously going to wonder about something in your story, you're obligated to at least acknowledge the question. Whether you answer it is up to you.