As the Bunker Project begins, Yun Tianming’s fairy tales lose public urgency. The IDC continues trying to decode them, but the work becomes just one PDC project among many. Some interpreters try to connect the tales to the Bunker Project, reading the umbrella as a defensive structure and the canopy’s stone spheres as the Jovian planets, but these interpretations mainly reassure humanity that its chosen path is correct.

AA visits Cheng Xin, who is studying modern science to catch up with the era’s advanced fusion-based technologies. AA demands an old-fashioned bar of bath soap, inspired by the princess in Tianming’s story. Because soap has long vanished from daily life, AA and Cheng Xin search a city history museum and buy an expensive preserved bar of Common Era soap.

Back at home, AA does not take a bath. Instead, AA asks Cheng Xin to fold an origami boat, another nearly lost Common Era skill. Cheng Xin remembers folding a paper boat with Yun Tianming in college and connects the image to the white sail at the end of his stories.

AA places a small fragment of soap into the stern of the paper boat and sets it in a bathtub. The boat moves by itself because the dissolving soap lowers the water’s surface tension behind it while the unchanged tension ahead pulls it forward. Cheng Xin instantly understands the physical principle and imagines the paper boat crossing space at light speed.

Cheng Xin then remembers the danger to Yun Tianming and the possibility, however uncertain, that sophons might still be observing the Solar System. She and AA deliberately suppress any visible excitement, remove the boat, and begin chatting about unrelated topics to conceal the importance of the discovery. In a carefully casual pause, Cheng Xin says only “Curvature—,” and AA silently confirms the conclusion: the fairy tale points to curvature propulsion.