Before heaven and earth existed, the third-century CE text Sanwu Liji relates, the universe was an amorphous chaos shaped like a great egg, in which the giant Pangu formed and slept undisturbed for eighteen thousand years. When he finally woke inside the darkness, Pangu split the egg open with a great axe he held; the light, pure elements suspended within it, the yang, rose slowly upward to become the sky, while the heavy, turbid elements, the yin, sank to become the earth. Fearing that sky and earth might drift back together and return the world to chaos, Pangu placed himself standing between them, his feet on the earth and his head supporting the sky, and pushed the two further apart.
As sky and earth continued to separate over the following eighteen thousand years, Pangu grew with them, adding ten feet to his height each day, so that by the time the two were finally fixed at a safe distance from one another, he had grown to an immense size, holding the entire structure of the world in the space of his own body until the separation was permanent and stable and his labor was finally complete.
The attribution of this account to the third-century compiler Xu Zheng means it survives only as a fragment quoted in a later encyclopedic work, the Yiwen Leiju, rather than as a complete original text, which is typical of how much early Chinese cosmogonic material has come down to modern readers in pieces rather than whole. Some strands of the wider tradition give the egg itself a color, black outside and yellow within, likening its shape and division to a chicken's egg, imagery scholars have connected to divination and cosmological diagrams that treat the egg as a compact model of the ordered universe still folded shut.