in c++ I need a pointer to a pointer ?????

Ok I am not the greatest in the world at pointers.

but I can build a linked list with no problems.

New problem is this I was parsing out some xml data
and I was trying my hand at recursion.

so to make it simple I took out all the parsing and just
did a little something to try and see if it works.

I did this

struct tree
{
        string data;
        tree *child;
        tree *sibling;
public:
    tree *Root_node;
    tree(){Root_node=NULL;}
    tree* add_node(tree*);
};

tree* tree::add_node(tree *root)
{
    int found;
    char str[120];
    string ReadData();

        string lineinput;
        static ifstream
input_line("c:\\testproject3.txt",ios::in);//input file
    while (!input_line.eof())
    {
    found = lineinput.find("</");
    tree *node = *root;
    root->data = lineinput;
    if (found >0)
        {
            root->child=add_node(root);
            root->sibling=add_node(root);
            return;
        }

    if (found <=0)
        return;
    }
}

=============================================
try as I might this worked but didn't.

Then I was told oh this wont work unless you use a pointer to a
pointer...something like **root.

Well I am lost and dont' know what to do this makes no since(to me)
and not sure what they are saying or how to do it.

can someone tell me what a pointer to a pointer does and if there is
something I can find to explain the syntax behind it?

thanks big time for help

Jerry