Hit the right emotional beat in a single, focused burst. The ones that stay with me don't try to cram a three-act structure into a paragraph. They pick a single moment, a precise emotion, and they twist it. Like that flash fiction 'The Last Question' by Asimov, sure, but there's a piece I stumbled on years ago about someone finding their childhood imaginary friend as an adult, just sitting on a park bench. It wasn't about the friend explaining anything. It was the adult sitting down, and the friend just saying 'You left.' That's it. The whole world built in four words after the setup.
What makes it stick is the white space it creates around the paragraph. The story is the ice cube; the memory is the cold water it floats in. It gives you the catalyst, the emotional punch, and then forces you to do the work of imagining the before and after. A full novel might over-explain that imaginary friend's origins. The paragraph just gives you the gut-punch reunion and trusts you to feel the decades of absence. The impact comes from its incompleteness, ironically. It's a perfect, sharp fragment.