Bilingual edition: English is preserved and Chinese follows each unit. Terminology uses the confirmed v20260916 glossary; automated semantic review remains traceable. 中英双语版：英文原文完整保留，中文紧随对应单元；术语采用 v20260916 确认表，自动语义审校结果可追溯。
Density and mass relationships
密度与质量关系
How do you measure mass and density? Does greater mass imply greater density? What quantity determines floating or sinking? Is it true to say that light objects float and heavy objects sink?