Revelation: Advise people to cherish time and don't wait until later to regret it. Cherish time, the past will never come again.

Original text:

This is the night when the lights are turned on at five o'clock. It is when a person is studying, he doesn't know whether he is diligent or not, and he regrets it too late when he turns white.

This is a famous sentence in the Tang Dynasty painter Yan Zhenqing's "Persuade to Learn".