The word 守口如瓶 is formed by sequentially combining four characters: 守 (shǒu, meaning "to guard"), 口 (kǒu, meaning "mouth"), 如 (rú, meaning "like"), and 瓶 (píng, meaning "bottle"). This arrangement creates a literal simile, "guard mouth like a bottle", where the impermeability of a bottle is used to metaphorically instruct keeping one's mouth tightly closed to maintain secrecy.