This Venn Diagram accurately represents my set of friends:

The large circle, X, would be me, and every other circle represents a set, consisting of one or more people who are my friends. The vast majority of people you meet you won't befriend, because, well, they suck. But every so often you meet a person and they become a friend and then they're assigned to a set.

Intersection of sets are transcribed as AX, which is transcribed in the above diagram by the shaded region. As you can see,

ABCDE = NULL

Each set is only mildly aware of the other. Sets don't mix.

An accurate real world example of this would be if I were to ever throw a huge house party, inviting all my friends. I'm sure sets would co-mingle with members from other sets for a while, talking about me of course. What else would they have in common? But as the night would progress, friends of the same set would draw closer together, away from the other members of other sets. Each set would eventually convene in seperate areas or rooms inside the house.

I hate parties.

***

You probably only ever make one or two real friends in highschool. The rest are just friends of convenience. Maybe you shared homeroom, or you walked together to school. But once that's over, and that everyday routine is gone, it's usually "so long friend".

XA = NULL

I have no desire to talk to the majority of people I knew in highschool. Even at the time I knew most were go-no-where idiots. If I do run into them now, this is usually the standard template for conversations:

(note: only my side of the dialogue is transcribed)

"So yeah, I went to university, finished my degree, and now I have a really cool job. what about you?"

"Hey I hear ya, school isn't for everybody. So what do you do nowadays?"

"Cool, security must be a fun job. Ever meet any rock stars at those concerts?"

"Yeah, I guess you wouldn't meet any rock stars on the job at Wal-Mart."

"So yeah, it was nice running into you again. See ya."

And as it usually goes, the people you would like to talk to again, you have no idea how to get in contact with.

***

Your friends of course have their own set of friends:

XE = NULL

Sometimes you meet them:

XDE = MEET (shaded)

You think E is cool and you like E. After a few more times, D = D + E.

D is now larger by sizeof(E).

***

X doesn't always know everything that happens in other sets:

The shaded region depicts B-X. X doesn't know what happens in B-X, or why. But, something does happen:

B is split into B and C. XB is still something, and XC is still something. But now, BC = NULL.