To know something like the palm of one's hand; to have an extremely thorough understanding.
了明白；指掌指着手掌。形容对事物了解得非常清楚，象把东西放在手掌里给人家看一样。
《论语·八佾》或问秂之说。子曰‘不知也；知其说者之于天下也，其如示诸斯乎！’指其掌。”何晏集解引包咸曰孔子谓或人言知秂礼之说者，于天下之事，如指示掌中之物，言其易了。”
After years of study, she knows the ancient texts like the palm of her hand.
Learn the next connected idiom: 了如指掌 (liǎo rú zhǐ zhǎng) → 掌上明珠 (zhǎng shàng míng zhū)
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