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

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


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!