And that's really neat to think about, because photons, of course, are massless particles, they have no mass, so it's neat to think about something that has no mass, but that actually does have a momentum.

而且那真的不容易想明白，因为光子，当然是无质量的粒子，它们没有质量，所以这个真的不容易想明白,一些物体没有质量，但是它们事实上确实有动量。

麻省理工公开课 - 化学原理课程节选

Let's turn to the next president who is gonna have a hard choice to make.

接下来我们谈谈下届总统选举的问题,这个不太容易搞定。

斯坦福公开课 - 经济学课程节选

It's worrying because you can't go away feeling comfortable about this.

因为你们不,容易接受这个

耶鲁公开课 - 博弈论课程节选