The test of a conclusion is in what the readers do after the paper ends, either what they do immediately or what they do over time.  Does the engagement of ideas go on, or does the reader put the essay down and go along unaffected?  Think of it this way. If the next few lines after your conclusion were to be written by the reader, what would you like them to say?

                                  I understand what you mean.

     Now there's a reaction to yawn about.  Understanding, even agreement, from the reader is a minimal goal.  There are even times when you don't want total understanding or agreement at first, such as when using touches of ambiguity properly to generate activity.  The traditional summary conclusion is usually about as pointless and boring as a conclusion can get, particularly in school essays that are only a few paragraphs long.  If you're writing a complex report of significant length, then a summary might be necessary to bring focus to the main ideas.  In the technical writing/business world, where such reports can be found, the summary may be the only part some readers look at.  However, in a paper of less than 10 pages, a summary is only necessary if you if the writing is unclear or your reader is particularly dense.  In either case, the best response is more revision.

     Sometimes you'll want a fairly quiet ending, like the easy closing of a door, firm and certain but not startling.  At times, this won't seem like an ending at all.  You've said what needs to be said for now, a meal when you've eaten just enough and dessert would be too much, a satisfied feeling as you begin to slowly and peacefully digest what you've taken in.  It's the difference between the ending of a paragraph or the end of a chapter, a sense of closure, though the entire story hasn't been told.  Actually, all writing is only part of the story, and the entire story with all its interconnections and branches can never be told.  When you've told enough, then stop.  If the ideas are written with enough depth and precision, the reader will continue thinking, perhaps fairly quietly in keeping with the tone of the writing, but in some depth and for some time.  No matter what the conclusion, bland writing, like bland food, is quickly forgotten.

     A variation on this conclusion with no conclusion can be like a slamming door or a slap in the face.  The writing is moving along when the climax to the story hits in a matter of one or two sentences.  No overt warning, no explanation.  You want the reader's head to snap back a bit, breath stop for a moment or two.  This type of ending gets an almost entirely emotional initial reaction.  If done properly, the emotional wave carries the readers into territory they may have been unwilling to enter otherwise.  It's a very manipulative technique, and if mishandled it can create resentment in the readers, but all writing is manipulative on some level.  Your choices are whether you want to manipulate the readers primarily through the subconscious or conscious, through emotion or reason, in an ethical manner or unethical.  While I come down in favor of ethics, the other two sets of choices are yours to make, and they probably need to be made for each essay, not as a blanket preference.

    I hadn't thought much about the slamming conclusion until a student came to her conference unable to find a conclusion for her essay.  She was writing about a time when she was about 10 years old, and a cousin of the same age drowned.  Her family spent the night before the funeral at the cousin's house, as they had many times before on happier trips.  So when my student woke up the next morning, at her cousin's house, her first, not yet awake thought was that they would get to play.  She was delighted and bounded down the stairs.  When she hit the foot of the stairs, she remembered that her cousin was dead.  The paper went on for another two or three pages, searching for a conclusion, a tidy explanation, but the conclusion was at the bottom of those stairs.  When realization hit her, it hit the reader.   There is no tidiness, no possible explanation that will do anything except lessen the impact.  

     Perhaps the best known essay with such an ending is "Once More to the Lake" by E. B. White.  It's certainly the most anthologized, and you should have little trouble finding a copy.  One warning.  Though critics talk about the impact of the ending, the essay isn't directed toward college-age readers, and I was well past 30 before the conclusion finally hit me.  Those of you still caught up in the sense of immortality youth provides may have to think about it.

    The trick is in setting up the effect so that the reader will find a sense of direction or purpose after the initial shock.  Tha element has to be embedded in the rest of the writing, done so the reader can't spot what's coming but will be moving in a particular direction.  They'll stagger when the conclusion hits, but if ou have the momentum built up enough, they'll stumble in the direction you want, then keep exploring that thought as they regain balance.  (Even when not using the slamming door, writing past the conclusion is a common problem.  If your essay seems never ending, look backward for your conclusion.)

     Keeping with the same motif, you can also write an opening door conclusion.  If there's a particular puzzle or quandry that you're trying to resolve, and the essay will untangle the matter, and give the reader an "Ah-ha" reaction at the end, then your're opening a door.  You aren't just passing on bits of information.  You're organizing and presenting ideas in a controlled manner in order to reveal something that was hidden or unclear.  No revelation at the end means you slipped up somewhere, either because there was nothing to reveal or because you haven't made it clear enough.  The reader should be able to move on to new ground, a new level of thinking, because of your ideas.  

     In the other conclusions, the reader has to stop first to consider what you've said.  The first reaction is a pause or full stop in order to absorb the ideas before moving on to application.  An opening door conclusion is like the first view of Oz in the MGM version of "The Wizard of Oz."  You've transported the reader to a new way of seeing and opened the door to that world, and they should be drawn across the threshold.

     Thresholds.  While I've emphasized the door aspect of the image, in many ways, it's the threshold itself that's important, that line between where we are and where we could go.  No matter whether the door is closed, slammed, or opened, what matters is getting the reader to cross the threshold at the end of your essay.  Actually, there are two thresholds because the introduction represents a crossing as well.  Each essay is a tunnel with a barrier at each end, and you must get the readers to cross each of them.  At the beginning, the barrier is the inertia your readers have, either inertia in another direction or the inertia of an object at rest, and you must get the readers moving with enough momentum to cross the threshold into your essay.  At the end, the readers must have escape velocity, the momentum needed to keep moving beyond your essay, to resist influences that pull them in other directions or back to a state of rest.

     Of course, you should also keep in mind that everything I've said is only partly true.  I've divided conclusions into three specific categories, but few conclusions will fit tidily into a category.  These are ways of thinking about the writing, not set forms.  Ask yourself questions about what the writing needs to do, develop attitude in the writing.  Most of the time you'll have a conclusion that mixes many possibilities, could be labeled in more than one way, but the only issue is whether or not that conclusion works with that particular essay in the way you want it to work.  The ending to the film "Sixth Sense" could easily be described as both a slamming door and an opening door, but when you look back and see how the story developed, there was no other ending that could have worked as well.  That's the definition of a good ending.