As you learned in Lessons 16 and 17, Director responds to events—mouse clicks, for example—by sending a message of the same name. The message travels through a specific path, searching for a handler with a name that matches the message.
When you click the mouse, Director first sends a mouseDown message followed (when you release the mouse button) by a mouseUp message. For the mouseUp message, Director looks for any mouseUp handlers assigned to the sprite you just clicked. If Director can't find a mouseUp handler among the sprite's scripts, it searches through the scripts assigned to the cast member associated with that sprite, to the frame in the movie that contains the sprite, and finally to the movie itself. If it can't find a mouseUp handler anywhere, Director discards the mouseUp message.