[jQuery] how i can get the "id" of the dropped tag

[jQuery] how i can get the "id" of the dropped tag

how i can get the "id" of the dropped tag in this code :
<script type="text/javascript">
$(document).ready(function(){
$("#draggable").draggable();
$("#droppable").droppable({
drop: function() {
          alert($(" ??? "));
        }
});
});
</script>
the tags is :
<ul>
<li id="item_1">
<li id="item_2">
<li id="item_3">
</ul>
thanks.