I've been playing this game for years and this is something I've never gotten a straight answer on or been able to figure out entirely for myself. When do you need to build a granary for it to have its full effect the first time a city grows after building it? What I mean is, if you finish construction on a granary the turn before a city grows, the city's food bar doesn't begin halfway filled like you might expect it to. It only "stores" a little bit of food. It's obvious that the granary must begin "storing" food after it's been built instead of simply causing the city to start with a partially full food bar each time it grows, though practically this would only make a tangible difference the first time a city grows after the granary is built.