守口如瓶 keep one's mouth shut like a sealed bottle
Explanation
比喻说话谨慎，严守秘密。就像瓶口紧紧地塞住，没有一点儿缝隙，秘密不会泄露出去一样。
The idiom means being discreet and keeping secrets, like a bottle that is tightly sealed and no liquid leaks out.
Origin Story
传说古代有一个名叫张三的人，为人正直，办事公正。一次，他被派去执行一项秘密任务，需要将重要情报送到远方。途中，他遇到了一群强盗，强盗们想要抢夺他手中的情报。张三为了保护情报，便将情报藏在了一个瓶子里，然后将瓶口紧紧地塞住。强盗们虽然搜遍了他的全身，却始终没有找到情报。张三凭借着自己“守口如瓶”的智慧，成功地完成了任务。
Legend has it that in ancient times, there was a man named Zhang San who was honest and just. Once, he was sent on a secret mission to deliver important intelligence to a distant place. Along the way, he encountered a group of bandits who wanted to steal the intelligence from him. To protect the intelligence, Zhang San hid it in a bottle and tightly sealed the bottle. The bandits searched him thoroughly but could not find the intelligence. Zhang San successfully completed his mission thanks to his wisdom of
Usage
用来形容说话谨慎，严守秘密。通常用于告诫别人要保守秘密，不要随便透露。
It is used to describe being discreet and keeping secrets. It is often used to warn others to keep secrets and not reveal them casually.
Examples
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他这个人守口如瓶，什么事都闷在心里，从不向别人透露。
ta zhe ge ren shou kou ru ping, shen me shi dou men zai xin li, cong bu xiang bie ren tou lu.
He's the type who keeps everything bottled up inside, never revealing anything to anyone.
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对待这个机密，你必须守口如瓶，不能泄露半点消息。
dui dai zhe ge ji mi, ni bi xu shou kou ru ping, bu neng xie lu ban dian xiao xi.
You must keep your mouth shut about this confidential matter and not let slip a single word.