Background
Cao Chong (196–208), courtesy name Cangsu, was born to Cao Cao and Lady Huan. His birth fell around the first year of Jian'an, precisely when Cao Cao was bringing the Emperor of Han to the capital at Xu — a critical moment for his rising political power. The San Guozhi places Sun Quan's gift of the elephant at the head of a passage describing Cao Chong as "shrewd and perceptive from an early age — at five or six his intelligence already equaled that of an adult." [c:16980] The corresponding date falls roughly between the fifth and tenth years of Jian'an. At this time Sun Quan had only recently inherited his elder brother Sun Ce's domain (in 200), and was paying his respects to the northern strongman as the young lord of Jiangdong. The enormous elephant, a southern exotic beast, was a gift laden with political symbolism: it displayed the wealth of Jiangdong's natural resources and hinted that north and south each had its own special advantages. Cao Cao's interest in the question of "weighing the elephant" was partly genuine curiosity and partly a gesture of responding to this political symbol by saying "I can find a way to measure your gift."
The Incident
The assembled ministers "could none of them produce a method" — with the weights-and-measures technology of the late Han, there was indeed no established way to directly weigh a living elephant. Cao Chong's solution was physically equivalent to the "substitution of equal buoyancy": place the elephant on a large boat, mark the waterline (determining the displacement), then replace the elephant with objects of known weight loaded until the vessel reaches the same waterline (the same displacement) — the total weight of those known objects equals the weight of the elephant. [c:16980] This is a practical application of Archimedes' principle of buoyancy. Although the Archimedean principle itself was known in the West from the third century BC, this record represents one of the early documented applications in China. The incident itself was small in scale and involved no military or political decisions, yet the Wei Shu records it with gravity as the opening entry in a systematic account of Cao Chong's many acts of "perceptive intelligence and benevolence," placed alongside incidents such as the "rat-gnawed saddle blanket" episode and his "arguing to save those condemned to death" — all part of a deliberate construction of the image of an unusually gifted child. [c:16980]
Outcomes
First: from that point on, Cao Cao "repeatedly praised Cao Chong before his ministers, indicating his wish to pass the succession to him." [c:16980] This record points to the direct influence of the Caochongchengxiang episode on the structure of the Cao succession question — Cao Chong's perceptiveness was repeatedly cited by his father, making him the de facto "undeclared candidate" among Cao Cao's sons. Second: the incident established Cao Chong's image within the Cao clan as "the exemplar of both wisdom and benevolence." The Wei Shu goes on to record that he repeatedly "explained the circumstances to Cao Cao," securing clemency for dozens of people facing severe punishment. This image of "perceptiveness combined with compassion" stood in sharp contrast to the overall political atmosphere of Cao Cao's later years, characterized by "strict and heavy enforcement" — Cao Chong was seen by his father as the best candidate to reconcile the tension between "strict punishment consistently applied" and "benevolent governance for lasting order." Third: the political meaning of Sun Quan's gift of the elephant was reversed once Cao Chong proposed his solution — what had been Jiangdong's gesture of "displaying wealth through an exotic beast" became instead the story of "a northern child's wit meets the challenge."
Significance
First: Cao Chong died in the thirteenth year of Jian'an (208) at age thirteen — Cao Cao "personally beseeched heaven and earth on his behalf," and after his death told Emperor Wen (Cao Pi): "This is my misfortune, but it is your fortune." [c:16980] This statement directly reveals the "clearing" effect of Cao Chong's death on Cao Pi's path to succession. Had Cao Chong not died young, the succession struggle between Cao Pi and Cao Zhi would very likely never have occurred — Cao Cao was evidently more inclined to pass the estate to this preternaturally gifted youngest son. Second: Caochongchengxiang, as the archetypal "childhood wisdom story" of Chinese antiquity, was repeatedly transmitted in later eras through primers such as Ershisi Xiao and Youxue Qionglin, grouped alongside Sima Guang smashing the water jar and Wen Yanbo filling the jar with water to retrieve a ball as the three canonical "precocious youths." Third: Sun Sheng, in his annotations to the San Guozhi, specifically cited the Spring and Autumn principle "establish the legitimate eldest, not the most worthy" in commenting on Cao Cao's desire to transmit the succession to Cao Chong, [c:16980] showing that this episode remained, in the eyes of the Six Dynasties historians, a political-ethical case still worth repeated deliberation — even if Cao Chong were far more gifted, advancing a younger son over an elder was still a breach of propriety. Cao Chong's death objectively spared the Cao clan from a genuine confrontation with this ethical dilemma, and it gave Jia Xu's counsel about "Yuan and Liu's sons" its natural context a decade later.