Inclusion of の when asking a question with 'why'


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?




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** ！

