I [came across](http://www.guidetojapanese.org/learn/complete/explanation) the
following sentence

> ところで、リー君はどうして忙しい **の** ？

translated as

> By the way, Lee-kun, why are you busy?

If the の is left out, what's the difference in 'feel' or implication? That is,
why is the sentence above different from

> ところで、リー君はどうして忙しい？

From what I've read, の seeks more explanation and context, but for a 'why'
question, is an explanation not already expected? What purpose does the の play
in the first sentence?
CC BY-SA 4.0
2018-05-16T23:07:19.533
58724
2021-07-31T21:05:35.530
27005
question
particle-の
Inclusion of の when asking a question with 'why'
の is a nominalizer in this case. Which means that it makes of the followed
proposition a noun.

If you know what 「のだ」 is, then you will not have any problem understanding
this one, because they are the same の.

> どうして忙しい？ : Why are you busy?
>
> どうして忙しいの？ : Why is it that you're busy?

That is why in some situations and with the right intonation, a question with
の may sound a bit girly, because it sounds more pushy than a question without
it.
CC BY-SA 4.0
2018-05-17T06:08:28.530
58734
2018-05-17T06:08:28.530
2020-06-17T08:18:27.500
-1
29939
58724
answer
You can use 「の」 to ask a question in japanese as simply.

For example:

> ご飯を食べている **の** ？

is meaning same as

> ご飯を食べています **か** 。

* * *

you also can ask reason with 「なぜ...の」. which is meaning same as 「どうして...んですか」

> **なぜ** 学校へ行かない **の**

* * *

give a order, very similar to 「しなさい」

> みんな話さないで、よく聞く **の** ！
CC BY-SA 4.0
2018-05-19T10:31:45.793
58819
2018-05-19T10:31:45.793
58724
answer