Has anyone noticed that greetings directed towards a person consist of 1 of 2 possibilities: "What's up?" and "How's it going?" There is only one correct response to each: "Not much" and "Pretty good." Under no circumstances do people actually want to hear how your life is going. Old people sometimes forget this. If you ask an old person how they're doing, they'll immediately tell you all about their bladder problems. Old men will also give you the latest news about their prostate. Old people's lives are generally centered around urine issues. A network could easily make an entire show that consisted purely of bladder jokes, and it would be an instant hit among old people.

Of course, sometimes you make the mistake of confusing the two greetings, which makes you look really stupid:

Friend: Hey. How's it going?
You: Not much.
Friend: (Pause) You're an idiot.

And there's also the timing issue involved with greetings. If you see somebody walking your way and you say hi to them too early, you've got that awkward pause afterwards until you actually pass each other

Friend (from far away): What's up?
You: Not much.
(Awful silence as the two of you keep walking towards each other.)
Friend: Did I already ask what's up?

As you've probably learned, the correct method of saying hi to somebody as you walk by them is to pretend you don't notice them until the very last minute.

You: (Really, absolutely fascinated by this spot on the ground that you've never noticed before)
Friend: Hey.
You: (Suddenly looking up as though their existence in this small hallway is a total surprise to you.) Oh! Hey! What's up?

Naturally, a lot of this only applies the first time you see somebody in the hall that day. The second time, you should only do that "mutual smile" thing (also acceptable is making a "Hey, you following me?" crack.) But God help you if you run into each other a third time - both of you should avoid any sort of eye contact whatsoever. You either have to pretend you don't see each other or do something regrettably drastic.