After King Wu of Zhou passed away, his son Song ascended the throne as King Cheng of Zhou. Since the Zhou dynasty had just pacified the realm and King Cheng was still young, the Duke of Zhou feared the feudal lords might rebel, so he acted as regent, handling state affairs and helping King Cheng suppress rebellions by Guan Shu and Cai Shu, earning the young king's deep trust. King Cheng's younger brother, Tang Shu Yu, obtained auspicious grain and presented it to the king, who then gave it to the Duke of Zhou as a gift. After seven years of regency, when King Cheng came of age, the Duke of Zhou returned power to him and stepped back into the ranks of ministers. King Cheng continued to respect him, frequently seeking his advice on various matters, and the Duke of Zhou offered frank counsel and loyal warnings without hesitation.
Once, after court was dismissed, King Cheng of Zhou was chatting with his younger brother Shu Yu. On a whim, the king plucked a paulownia leaf, carved it into the shape of a jade tablet, and gave it to Shu Yu, saying, "I will enfeoff you with this." Shu Yu took it seriously, was delighted, and told the Duke of Zhou about it.
When the Duke of Zhou heard this, he came to consult King Cheng: "I hear Your Majesty intends to enfeoff Shuyu. Please select an auspicious day to establish him." King Cheng replied, "I was merely joking with Shuyu."
The Duke of Zhou said solemnly, "I have heard that the Son of Heaven makes no jest; every word he speaks is recorded by historians, sung by musicians, and praised by scholars far and wide. If the Son of Heaven speaks carelessly and loses trust, he will lose the people's hearts."
King Cheng, heeding the Duke of Zhou's advice, realized his words were improper and later formally enfeoffed his younger brother Shu Yu in the land of Tang, located east of the Yellow River and Fen River, spanning a hundred li, thus earning him the title Tang Shu Yu. Shu Yu's son Xie, noting the Jin River south of Tang, renamed the state Jin. In the early Spring and Autumn period, Jin split in two before being reunified by Duke Wu of Jin. Through generations of effort, Jin grew increasingly powerful, eventually becoming one of the Five Hegemons of the Spring and Autumn period.
The Duke of Zhou was a master of persuasion, but King Cheng of Zhou was even more adept at heeding his ministers' advice, carefully weighing his every word and action. Following the plans laid out by his father, King Wu, King Cheng had the Duke of Zhou conduct repeated inspections to build the city of Luoyi. There, he moved the Nine Tripods—symbols of imperial power—to receive homage from the feudal lords. He also relocated the remnants of the Shang dynasty to Luoyi, revised the rites and music to restore order, and pacified the people. As a result, during King Cheng's reign, the realm was peaceful, the people lived in harmony, and it was a golden age of prosperity.
Later, the idiom "The emperor's words are no joke" came to mean that a ruler must speak carefully and not make frivolous remarks.
Source: *Lüshi Chunqiu*, Chapter "Weighty Words"
Meaning of the Idiom: Later, the Chinese idiom "天子无戏言" came to describe a ruler must speak carefully and not make frivolous remarks.