non-static class cannot be referenced from a static context

Possible Duplicate:
Why do I get “non-static variable this cannot be referenced from a static context”?

Here are the codes

public class Stack
{
    private class Node{
        ...
    }
    ...
    public static void main(String[] args){
         Node node = new Node(); // generates a compiling error
    }
}  

the error says:

non-static class Node cannot be referenced from a static context

Why shouldn't I refer the Node class in my main() method ?