One thing that I want to mention that is really important is once you have double bonds, what happens between those two atoms in the molecule is they can no longer rotate in relation to each other.

我要提醒你们的非常重要的一点事,一旦有了双键，分子中的两个原子,就不能相对旋转了。

麻省理工公开课 - 化学原理课程节选

The number that you choose to rotate your letters by is what we call in cryptography, a key.

你选择的旋转字母的数量,就是所谓的加密学，一个密钥。

哈佛公开课 - 计算机科学课程节选

So in order to rotate a double bond, you have to actually break the pi bond, so essentially what you're doing is breaking the double bond.

为了能够旋转双键，你必须打破一个π键，本质上我们要做的就是打破双键。

麻省理工公开课 - 化学原理课程节选