This is a total noob question.

I am reading [Naive Set Theory by Paul R. Halmos](http://www.amazon.co.uk/Naive-Theory-Undergraduate-Texts-Mathematics/dp/0387900926), and I'm having difficulty to understand something which seems to be trivial.

In the first chapter he writes:

> 
> If $x$ belongs to $A$ ($x$ is an element of $A$, $x$ is *contained* in $A$), we shall write
> 
> 
> $x\in A$
> 
> 
> 

I understand this.

Then, he write:

> 
> If $A$ and $B$ are sets and if every element of $A$ is an element of $B$, we say that $A$ is a *subset* of $B$, or $B$ *includes* $A$, and we write:
> 
> 
> $A \subset B$
> 
> 
> 

I understand this too.

Then he says:

> 
> The working of the definition implies that each set must be considered to be included in itself ($A \subset A$); this fact is described by saying that set inclusion is *reflexive*.
> 
> 
> 

I understand this too.

But then:

> 
> Observe that belonging ($\in$) and inclusion ($\subset$) are conceptually very different things indeed. One important difference has already manifested itself above: inclusion is always reflexive, whereas it is not at all clear that belonging is ever reflexive. That is: $A \subset A$ is always true; is $A\in A$ ever true? It is certainly not true of any reasonable set that anyone has ever seen.
> 
> 
> 

*And this is where I don't think I understand anything.* There is not more elaboration on this point in the text.

I tried to skip this but it seems it is quite fundamental for understanding what follows in the book.

Could someone explain what is meant here?