the.com/punch line

The sentence that makes the first ten worth it.

means The final phrase of a joke or story that delivers the twist and makes it funny.

from A late-19th-century American coinage pairing 'punch' (the hit or jab) with 'line' — the words a comedian or storyteller lands like a blow after building you up. 'Punch' had already carried the sense of forceful impact for centuries; 'line' narrowed it to the exact scripted words doing the hitting.

origin
from boxing, the final blow that lands
structure
needs setup to steal all the credit
timing
arrives late on purpose, that's the whole trick
misfire risk
explained punch lines die instantly, every time
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