Limit drag area of scriptaculous draggable

Question

Im usign this to create a bunch on draggable elements:

$$('.box').each( function(item){ new Draggable(item) });

How can I limit the draggable area? This way, the boxes can be droped anywhere on the page, and I would like to limit the drag area to their current parent element (I dont think I need to create a droppable). Or, at least, i would like to revert the box to its initial position if its dropped outside it parent. How can I do this? thanks.