[jQuery] Why this simple code doesn't work ?!?

[etnas]

Hi, I have this simply code and I don't know where is the problem:

The script:
**************************************
$(document).ready(function(){
        $('#button').click(function(){
                var data = '<input type="button" value="push"
class="new_button"><br />';
                $('#test').append(data);
                lets_go();
        });
});

function lets_go(){
        $('.new_button').click(function(){
                alert ('hello');
        });
}
***********************************

And the HTML:
***********************************
<input type="button" id="button" value="make a new button" />
<br /><br /><br />
<div id="test"></div>
************************************

Ok, that's all.
Why when I make a few buttons and click, the alert-box pops as many
times as buttons are below?

Test on Firefox.

How can I resolve this??

Thanks a lot!