Write a story that begins and ends with the same sentence, but means something entirely different each time.
This is a structural prompt, and structural prompts are some of the most useful exercises a writer can do. You commit to a single sentence in two positions, then everything between them has to earn the shift in meaning. The reader leaves the first sentence with one set of associations and arrives at the second sentence with another. That gap is where your story lives.
It works because it forces you to think about context as meaning. The same words can be a promise or a threat depending on what came before. Try a domestic line — "She left the keys on the counter" — and write a story that makes it sound peaceful at the start and devastating at the end. Or reverse it. The constraint will pull a story out of you that you would not have written otherwise.