First, click on the set variable line, and in the Variable: box on the right, type /:selection.
In the Value: box type clip1.
This variable, called selection, will now have the value clip1 whenever the mouse is clicked on the thumbnail.
The secret of dragging the thumbnail clip outside of the window is that, we donΓÇÖt actually drag the thumbnail anywhere. Instead, we have a movie clip on the main timeline, sitting outside of the stage area, so the user canΓÇÖt see it. This movie clip contains all thirty-one thumbnails.