Question: The question mould is:

> 
> How does X work? 
>  I've read the [*Wikipedia article / original paper / documentation / other resource*] but I can't make sense of it. Please explain X to me in plain English. 
> 
> 
> 

where X is a concept, algorithm, pattern, etc. (Not a code dump.)

I've come across a number questions fitting this mould. I never really know how to react. The result could perhaps be quite useful, but it doesn't quite seem to be a natural fit for Stack Overflow as a Q&A site. 

One reason is, it isn't clear what the appropriate scope of a good answer would be:

* Explain which part? (Some of it? All of it?)
* To what level of detail?

The appropriate result is ill-defined and unbounded: could be anything between a one-liner and a 100-page treatise. 

Example\*:

* [How does Dijkstra's self-stabilizing algorithm work?](https://stackoverflow.com/questions/6435046/how-does-dijkstras-self-stabilizing-algorithm-work)

Questions:
----------

* What is the appropriate reaction to such a question?
* When is such a question valid?

\* Feel free to edit this if you want to add more examples.

Answer: 
You can vote to close it as "unclear what you're asking" if the scope is ill-defined, or "too broad" if the question is decidedly so. Prompt the questioner to add specifics as appropriate.

Such a question can be valid provided the asker has demonstrated research effort, the question is narrowed down enough and the subject matter is on-topic for our site.
"**Explain X to me"** is no different from "**Why does this code work/does not work?**". Your reaction should depend on whether additional information is included in the question, i.e. whether any research was made prior to posting this question.

I would argue that including links in the question constitutes research. You can google for your key phrase and include top 5 links, using 1min of your time.

Let's consider the question you linked as an example:

> 
> I have read his seminal paper, Self-stabilizing systems in spite of distributed control. However, I don't quite get how the self-stabilizing algorithm works. I am most interested in his, **'solution' of k-state machines**. The density of the paper is quite intense and I can't make much sense of it. How does this algorithm work in plain English?
> 
> 
> 

I highlighted the part which suggests that OP has in fact done some research. However, I think it is not enough to make a good question. Whether such a question should be answered is up to the community.

I would expect the OP to explain a bit more of what they understand, and what they don't, from the topic they want explained. My reasoning is simple - if they cannot provide a detailed explanation of what they don't understand, there is a high chance they will not be able to provide feedback on the answer (i.e. keep asking questions in comments, without ever getting to the point).