For the toddler, the answer is very easy: "Because." Often, that can be expanded: "Because if we don't eat all our dinner, we can't have ice cream later." Because if we don't pick up the toys, Daddy will step on them, and they will break." Cause...Effect.
It works that way with writing, too. It's easy to get into the writing of the story, tripping along our merry little way, and forgetting that as the author of the story, we have many details and visions in our heads that the reader is, obviously, not privy to.
My crit partner, AKA "Sherry MeaniePants," is fond of the "Why?" question. "I see you have X happening, and it's a good event, but WHY?" or "H reacted this way to Z stimulus, but WHY?" or "Why did Secondary Character do W? Why?"
My stock answer is, "Because." (Bet you didn't see that one coming!)...And after I go back and read the section that started the spat of Why? questions, I can expand. "Because, her parents X,Y, Z'd, and this is her way of dealing with it, and it looks like I'd better make that clear in writing, because, yanno, that whole thing about readers not being able to read my mind."
I spent yesterday reading some e-books I bought online. Two I really enjoyed (other than some language in the first one I read that I didn't like, because I find nothing erotic about the slang words involved for describing various body parts, I enjoyed the story, and skipped over the objectionable words)...the third one, well, the premise was interesting, it did get my attention with the back cover blurb, but the actual story? I dunno. A couple of times, after I "turned" the page, I had to go back to see if I'd somehow jumped 3-5 pages ahead. It left me asking WHY? (Which Sherry MeaniePants found hysterically funny---I think she's STILL laughing at me, the beeyotch. ;) )
But the thing is...I'd really rather be pestered with loads of WHY's? as I'm writing the story. This is something that is within MY ability to fix before I send the story off. Why (hehe) give the editor an opportunity to reject a manuscript riddled with holes, unresolved issues and confusing transitions?"
It's also a good tool to use in the transitions at the end of chapters or in the transitions between scenes within the chapters. After all, you want the story to flow nicely, rather than leap about and wear out the reader or leave them wondering just what they missed.
All Write With Coffee: Writing Transitions and Tie-backs: Part III (And Part I and Part II)
Go With the Flow: Write With Transition Words and Phrases
To me, the greatest pleasure of writing is not what it's about, but the inner music the words make. ~Truman Capote, McCall's, November 1967