Well aren't you a smart cookie then.

哦，那你肯定是一个聪明人咯。

I can tell you're one smart cookie.

我能看出来你很聪明。

Yes, you were. Well done. You are a smart cookie!

是的，回答正确。做得不错。你真聪明！

So is 'smart cookie' just an American expression?

那么“smart cookie”只是美国人的说法吗？

A smart cookie refers to someone who is really clever.

聪明的小甜饼指的是非常聪明的人。

A smart cookie is a person who makes good decisions.

聪明人指做出正确决定的人。

Let's see if you're a smart cookie.

让我们看看你够不够聪明。

He was able to do it, so obviously he's a pretty smart cookie.

他能做到，显然他是个很聪明的人。

If you're a smart cookie, then you might be the teacher's pet.

如果你是一个聪明人的话，那老师肯定很喜欢你。

And the last one: If someone is very clever, we could say they are one smart cookie.

如果某人非常聪明，我们可以说他是个聪明人。

We know you're all smart cookies so we expect it was a piece of cake.

我们知道你们都很聪明，所以我们希望这节课的内容就是小菜一碟。

You may picture a smart cookie hiding in their study, ingesting hundreds of books, solving problems, and creating new inventions.

你可能会想象智者躲在书房里，翻阅数以百计的书籍，解决问题，并创造新发明。

" She's a smart cookie. She knows exactly what he's trying to do" , etc.

所以：“她是个聪明人。她很清楚他想做什么” ，等等。