When a clip is dropped onto the timeline, some scripting inside the above button script detects whether this clip has been dropped onto another movie clip. 

Note: you can only detect where things have been dropped on movie clips!

So how do we detect what has been dropped where? Well fortunately there is a built in property inside flash that will tell us all we need to know: _droptarget
Before we detect the drop, we need to give all the frames the correct instance names. I named them frame1, frame2 etc for all thirty-one frames.