This writing technique is called chiasmus, where the structure of the second half of the sentence is the mirror image of the first. In the same speech, Kennedy also said, “Mankind must put an end to war, or war will put an end to mankind,” and, “Let us never negotiate out of fear, but let us never fear to negotiate.” Again, chiasmus.

Chiasmus tickles our brains because it appeals to our natural affinity for symmetry. It’s the Taj Mahal of sentence structure, the Cologne Cathedral of one-liners — both symmetry marvels.

For Kennedy, the chiasmus shifts the focus from a self-centered perspective (“what your country can do for you”) to a collective, altruistic one (“what you can do for your country”). The flip makes you rethink your relationship to your country and your contribution to it. This inversion jolts you into considering a deeper meaning behind the words.

Chiasmus turns writing into song. It shows how small changes in the structure of a sentence can create an order-of-magnitude difference in the reader’s ability to remember it. The original sentence wouldn’t have been nearly as memorable if JFK had said: “You ask what your country can do for you, but maybe it’s time for you to contribute.” And he also would’ve sounded like a nagging mother who wants her kid to help with chores around the house.

Use chiasmus to craft geometric one-liners that’ll make your message elegant and enduring. And if you choose to run for President, use it whenever you want to give the TV producers a clippable moment.