Although face masks, which have become a common sight in Mexico City, are worn by only a few people on the streets of Juarez,pharmacies say they are doing a brisk business selling them.

VOA: standard.2009.04.30

The power to know is our ability to recognize, by sight, members of the same polis or city.

求知的能力,是我们透过双眼辨别，同一城邦或城市成员的能力。

耶鲁公开课 - 政治哲学导论课程节选

Mathematically, it's done by taking the expression for R and differentiating everything in sight that can be differentiated as vector t.

从数学上看是这样的,写出 R 的表达式,并对所有可以进行矢量微分的部分,对时间 t 求微商

耶鲁公开课 - 基础物理课程节选