About ahah in form

Hi, about ahah in form. i'm new in this part and i found it really hard to make it work.

Below is the code:
$form['xxx']['yyy'] = array(
'#type' => 'select',
'#title' => t('title'),
'#default_value' => $id,
'#options' => $arr,
'#description' => t('The desc.'),
'#required' => TRUE,
'#ahah' => array(
'event' => 'change',
'path' => 'menu/path',
'wrapper' => 'the-div-wrapper',
'method' => 'replace',
),
);

$form['xxx']['zzz'] = _function_to_get_element($id); // this is also a selection element

The logic is when i select the 'yyy' element, i want to update the 'zzz' element content according to the 'yyy' selection value.

I tried many times, when i change 'yyy' selection, nothing happens. I don't konw why.

Any suggestions appreciated!
Thanks!