[jQuery] droppable limit

[pEeLL]

Hi

i use for drag and drop this easy code

$(function() {

                $(".ui-draggable").draggable({ revert: 'invalid' });

                $(".ui-droppable").droppable({
                        accept: '.ui-draggable',
                        drop: function(event, ui) {
                                $(this).addClass('red');
                        },
                        out: function(event, ui) {
                                $(this).removeClass('red');
                        }
                });
});

i have some boxes and some containers. i  want add ONLY one box to one
container. How can i limit number of boxex that will container
accept?  thanks for every help.