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And lastly, here's Destin, from "Smarter Every Day,"
最后是《每天聪明一点点》的德斯丁，
talking about the conservation of angular momentum, and, since it's YouTube, cats:
为我们讲述动量守恒，由于是在YouTube上，所以他用猫来举例：
Hey, it's me, Destin. Welcome back to "Smarter Every Day."
嗨，我是德斯丁，欢迎再次收看《每天聪明一点点》。
So you've probably observed that cats almost always land on their feet.
也许你们注意到了，猫在落地的时候总是脚先着地。
Today's question is: why? Like most simple questions, there's a very complex answer.
今天我们就来讲讲这是为什么。大多数简单的问题都有一个异常复杂的答案。
For instance, let me reword this question:
所以，我来换一种问法：
How does a cat go from feet-up to feet-down in a falling reference frame,
为什么猫在一个下降坐标系中，可以将四脚朝天的姿态调整为四脚着地，
without violating the conservation of angular momentum?
而又不违反角动量守恒呢？
So, here's something all four of these videos have in common:
这四个视频有一些共同之处：
They all have more than half a million views on YouTube.
它们在YouTube上的观看数都超过了50万。
And those are people watching not in classrooms,
虽然观看的人没有坐在教室里，
but because they are part of the communities of learning that are being set up by these channels.
但他们都是学习社团的一部分，而学习社团就是由这些（YouTube）频道组成的。
And I said earlier that YouTube is like a classroom to me, and in many ways it is,
我之前说过，YouTube对我而言就像教室，
because here is the instructor -- it's like the old-fashioned classroom:
因为它跟教室很像，有老师在讲课，就像传统的课堂：
here's the instructor, and then beneath the instructor are the students, and they're all having a conversation.
老师在前面讲，学生坐在下面听，而且他们还可以交流。
And I know that YouTube comments have a very bad reputation in the world of the Internet,
我知道YouTube的评论在互联网世界里名声非常不好，
but in fact, if you go on comments for these channels, what you'll find is people engaging the subject matter,
但实际上，如果你去看这些频道的评论，你会发现人们真的在讨论视频的内容，
asking difficult, complicated questions that are about the subject matter, and then other people answering those questions.
他们会针对视频内容问各种复杂的问题，其他网友则会回答这些问题。
And because the YouTube page is set up so that the page in which I'm talking to you is on the exact
因为YouTube页面是设定好的，所以我刚刚讲一战的那个网页，
the place where I'm talking to you is on the exact same page as your comments,
会和你们发表的评论，在同一个页面出现，
you are participating in a live and real and active way in the conversation.
你们参与的是一个实时的、真实的、活跃的对话。
And because I'm in comments usually, I get to participate with you.
因为我也经常发表评论，所以我也跟大家一起在参与。
And you find this whether it's world history, or mathematics, or science, or whatever it is.
这些（评论）有关于世界历史的，关于数学的或者科学的，包罗万象。
