jquery droppable accept

Question

Can anyone tell me how I can write a function in accept condition and then how does it finds out that what to accept and what not to accept. For example, I want to accept

Answer 1

This is my solution:

var foo = true;

$( ".draggable" ).draggable({ 
                        revert: function(){if(foo == true){return true;}else{foo = true;}},
                         });

    $("#droppable").droppable({
        activeClass: "ui-state-hover",
        hoverClass: "ui-state-active",
        drop: function( event, ui ) {
            if($this == $that){
                foo = true;
                alert("this will revert the draggable back to original position");
            }else{
                foo = false;
                alert("this will NOT revert the draggable back to original position");
            }
        }
    });