Bottle conditioning carbonates beer naturally, inside the sealed bottle itself, by adding a small, precisely measured dose of priming sugar before capping — residual yeast still present in the beer ferments that sugar, producing CO2 that has nowhere to escape and dissolves into the beer instead, building carbonation over roughly one to a few weeks at room temperature.
Precision in the priming dose is the whole safety mechanism
Because the fermentation happens in a sealed container with no pressure-relief valve the way a keg has, the priming sugar amount needs calculating carefully rather than estimated by eye — run your batch volume, temperature, and target carbonation through the priming sugar calculator rather than guessing, since too much sugar risks over-carbonation and, at the extreme, actual bottle bombs.
Why residual gravity from a fruited or sweetened batch complicates the math
A standard priming calculation assumes the beer has already finished fermenting and has no meaningfully fermentable sugar left beyond what's deliberately added — a cider or beer that still carries real residual sweetness (from an unfermented fruit addition, an unstabilized sweet finish, or a stuck fermentation) can keep fermenting further in the bottle on top of the priming dose, risking dangerous over-carbonation that a standard calculation never accounted for.