了如指掌 know like the back of one's hand
Explanation
形容对某件事情非常了解，就像东西在手掌里一样清楚。
To describe someone's thorough understanding of something, as if it were in the palm of their hand.
Origin Story
话说春秋时期，鲁国国君想举行隆重的祭祀仪式，他向孔子请教祭祀的具体礼仪和步骤。孔子对鲁国的情况了如指掌，他指出鲁国的宗庙制度存在问题，与周朝的礼仪不合，并详细地解释了周朝祭祀的礼仪。他侃侃而谈，对各种细节都解释得清清楚楚，仿佛所有祭祀的程序都在他手心里一样。国君听得入神，对孔子的学识钦佩不已。后来，人们便用“了如指掌”来形容对事情非常了解，就如同把东西放在手掌心里一样清楚明白。
During the Spring and Autumn period, the ruler of Lu wanted to hold a grand sacrificial ceremony. He consulted Confucius about the specific rituals and steps. Confucius, who was intimately familiar with the situation in Lu, pointed out that the temple system in Lu had problems and did not conform to the etiquette of the Zhou Dynasty. He explained in detail the rituals of the Zhou Dynasty's sacrifices. He spoke eloquently, explaining various details clearly, as if all the procedures of the sacrifice were in his palm. The ruler listened intently, greatly admiring Confucius' scholarship. Later, people used “liǎo rú zhǐ zhǎng” to describe someone who has a complete understanding of something, just like having something in the palm of their hand.
Usage
用于形容对某事物非常了解。
Used to describe a thorough understanding of something.
Examples
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他对公司的情况了如指掌。
tā duì gōngsī de qíngkuàng liǎo rú zhǐ zhǎng
He knows the company's situation like the back of his hand.
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他把整个事件的来龙去脉了如指掌，我们对他都非常敬佩。
tā bǎ zhěnggè shìjiàn de láilóngqùmài liǎo rú zhǐ zhǎng
He knew the ins and outs of the whole incident like the palm of his hand, and we all admired him greatly for it.