custom controller not found [ci 2.0.3 php5]

[eluser]Unknown[/eluser]
I'm trying to create a new controller in ci 2.0.3 with php5.
I called it 'Site_controller' and put it in 'application/core' folder.
It simply contains this:

Code:

class Site_controller extends CI_Controller {
    function __construct() {
        parent::__construct();
        echo "TRYING...";
    }
}

In'application/controller/site/site.php' i'm trying to extend it so:

Code:

class Site extends Site_controller {
    function __construct() {
        parent::__construct();
    }

But it doesn't work. I can make it work only copying Site_controller file in 'application/controller/site' and including manually the class file so:

Code:

include("Site_controller.php");
class Site extends Site_controller {
    .
    .
    .

What's wrong???