Lü Meng was a general of the state of Wu during the Three Kingdoms period. Born into poverty, he received no formal education in his youth and had little time for reading even after joining the military.
Once, the King of Wu summoned Lü Meng and advised him, "Now that you are a general in command, you should read more to broaden your horizons."
Lü Meng replied, "I am too occupied with military affairs to find time for reading."
The King said, "Are you busier than I am? I still make time to study military texts and benefit greatly from them. Learning from the wisdom of our ancestors will help you improve."
Taking the King's advice to heart, Lü Meng devoted himself to studying history and military strategy. One day, Military Governor Lu Su visited Lü Meng and was astonished to find him remarkably knowledgeable. Lu Su, who had previously held Lü Meng in contempt, exclaimed, "I never expected you to be so learned beyond military matters! You are not the man you used to be!"
Lü Meng responded, "One should look at a person with a new eye even after a separation of just three days."
This conversation gave rise to the idiom Guā mù xiāng kàn (or Guā mù xiāng dài). It is used to express astonishment at someone's fabulous progress or transformation, implying that one should renew their respect for that person.