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I've been running into this trailing の particle quite often and can't seem to
understand what exactly it means. Is it replacing か, or does it mean something
else?

For example:

> まだ生{い}きているの？
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2013-10-17T04:50:28.953
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particle-の questions sentence-final-particles
Meaning of trailing の in a question
"Instead of 「か」, real questions in casual speech are usually asked with the
explanatory の particle or nothing at all except for a rise in intonation"

<http://www.guidetojapanese.org/learn/grammar/question>
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2013-10-17T05:25:51.440
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To add to what 無色受想行識 said, の is regularly used in conjunction with
interrogative words, such as in どこに行くの？or 何を食べるの？ Often when no interrogative
word is used, rising intonation will mark that the sentence is a question,
such as in 明日、パーティに行く？

Additionally, の can be used to indicate surprise or disbelief. For example,

```

    A: 明日、パーティに行くよ。
    B: 行くの？ 
    
```

In this instance, B probably thought A wasn't going to the party, and is
surprised to learn that he is.
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2013-10-17T06:26:57.950
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To put it as directly as possible, based on my experiences and learning, the
best way to describe the article の appearing at the end of questions, after
verbs, is to convey a stronger sense of curiosity than simply asking the
question without it. If you're familiar with the んですか？[noun]なんですか？
construction, it plays a similar role.

You might think of it as:

> お手洗いはどこありますか？ ＝ Where's the bathroom?  
>  お手洗いはどこあるの？あるんですか？= Where's the bathroom?(I really need to go!)  
>  
>  何をしている？= What are you doing?  
>  何をしているの？= What in the world are you doing?/What are you doing?(Tell me! I
> wanna know!)

(Disclaimer: This isn't a super natural phrasing for this question. It was
bent to fit my example. お手洗いはどこですか？ is much more common.)
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