Yabla video uses of 起落

原始 yuánshǐ
rén
通过 tōngguò
观察 guānchá
太阳 tàiyáng
de
周期性 zhōuqīxìng
起落 qǐluò
有了 yǒule
de
概念 gàiniàn
Primitive peoples observed the sun's cycle to come up with the idea of day.