When you're teenagers, you don't quite know what to write on there. Little Os and Xs for kisses and things like that.

VOA: special.2010.02.08

You know when you add 2 polar forms, you add the radii together, you don't add the angles together, you need to do it in Cartesian form.

表示形式的不同理解，返回给你了不同的值，这也就带来了一系列的问题，第二个问题是。

麻省理工公开课 - 计算机科学及编程导论课程节选

Homework is when you really figure out how much you know and don't know.

作业能让你真正弄清楚,自己对知识的掌握程度

耶鲁公开课 - 基础物理课程节选

You are so competitive, you want to win so badly and when the adrenaline starts, you don't know when to stop.

BBC: SPORT | Tennis | Blake warns Murray about fitness

"That's what you call it when you don't know what else to call it, " says David Clarke, a gastroenterologist in Portland, Ore.

WSJ: Why Your Heartburn Drugs Don't Work

But, "when you don't know the answer, refer them to someone who does and know when it's not our place to say something, " says Ms. FitzPatrick.

WSJ: Giving More Support to Support-Group Leaders