The arrows in this drawing show the relationships among the functions—which functions talk with each other or pass each other messages of one sort or another.
	You’ll notice that one of the arrows is bi-directional;
	it indicates that some sort of request and response conversation goes on between the two functions it joins.
	You can also see that two  functions interact with other functions outside the boundaries of this story.
	What is the nature of all these interactions?  They all have this in common: the functions communicate with each other by passing information to each other. The arrows show the direction the information travels.
	Let’s call the individual packets of information “messages”.
	I’ve added the names of the messages to the next slide.