There was a recent discussion on #famous IRC channel about the usage of a draggable and the lack of examples. That day I decided I should make one. A draggable is a very neat little modifier that allows you not only to drag some surface around, but also provide you with it's accurate position. You could then use this info to do some interactions with your other surfaces, e.g. if you are dragging it over another surface, the latter could transition to the place of the original one, effectively swapping the places.