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

So I'm actually writing my own fiction, and no, it's not easy. But you can actually learn from, if you read other writers.

我现在也正在写自己的小说，真的不容易。但是你可以从你以前阅读的作品中学到很多。

写作课程比较难 - SpeakingMax英语口语达人

I should be able to do nice bell-shaped curves, but it's harder than it looks standing up here; so that's your bell-shaped curve.

我还是挺擅长画钟型曲线的,但站着画真的不太容易,这就是钟型曲线

耶鲁公开课 - 金融市场课程节选

That's really hard to do though, you know.

但这真的很不容易做。

想成为配音演员 - SpeakingMax英语口语达人