The omitted detail was what made the joke funny.
A story is built like a very long joke, and in order to arrive at the punch line we usually need to know some details that come from a time prior to the chosen starting point.
Those details make up the backstory.
As the story moves along we present scenes that unfold in the real-time of the story. We can do the same and flashback to scenes in the past creating a little eddy, a hiccup in the sense of exactly where “now” resides in the story.
We can also strive to get the needed information across without creating a rift in the space-time continuum. Here are some ways to do that.
Keep it brief. Don’t divert into a full-blown scene unless it is absolutely necessary, and whatever you do, don’t jump from one scene in the story’s past to another scene also in the past.
Moving from the story’s now, to an earlier time, then another earlier time that may be before or after the first earlier time–but not quite up to the now of the story–need I say this is too disorienting?
Use dialogue to convey the information. “Yeah, it took me five years, but I finally got my GED last month.” Dialogue spoken in the story’s present fills in backstory without jolting the reader back in time.
Use summary. It sounds completely different from a scene and does less to disturb the timeline: Before moving to Detroit, Johnson lived aboard a rusty freighter bound for nowhere in particular.
Keep backstory brief, giving just what is needed. If you feel you have to loop the timeline and build a full-blown scene from the past, pay particular attention to the transitions into and out of that past scene. A simple line break can alert the reader to the change.
If you cue the reader with lines like: Joan was amazed to find herself still in the dentist’s chair and not aboard Lester’s yacht, you have stayed away from the scene you left for backstory for too long. Don’t make your reader feel as if they are waking up in an unfamiliar room.
The “present” of the story should be clear and strong to the reader, backstory used only when needed to inform that present.