Pictographic character “凹凸”

A rock falls from the sky and left a crater in the ground. Upon this phenomenon, ancient Chinese people created the pictographic characters, "凹" (āo) and "凸" (tū), which mean "concave" and "convex" respectively.

（汉字动态演示：）

巨石陨落，砸出坑洞。中国古人根据这一现象，造出了象形字"凹" (āo) 和"凸" (tū)。

"凹" describes the surface indentation of an object vividly, which draws its meaning "a lower level than the area around". Opposite to "凹", "凸" describes the surface projection of an object, which concludes the meaning "a higher level than the area around". 

"凹"是物体表面凹陷的形象描述，由此产生低于周围的字义。"凸"是"凹"的反义词。指的是物体表面突出的一块，由此产生高于周围的字义。

"凹" and "凸" can be combined into a word to mean "uneven" or "bumpy". For example, we can say, "这段路凹凸不平，不好走 (zhè duàn lù āo tū bù pínɡ, bù hǎo zǒu)", which means that "it is a hard road, bumpy and rough".  

因此，"凹凸"可组成词语，用来形容高高低低的，不平整的物体。我们可以说，"这段路凹凸不平，不好走。"(zhè duàn lù āo tū bù pínɡ, bù hǎo zǒu.)