For example, a lot of conspiracy theorists who believe the Pentagon was hit by a missile or truck bomb have pointed out that a 757 has a 124 foot wingspan and have asked how it could fit into a hole that was only 90 feet across. Long story short, per the Popular Mechanics Debunking 9/11 Myths book, part of the wings were likely sheared off when the plane hit the very thick walls of the Pentagon.
A much better question, which the conspiracy theorists would need to answer would be, "If the Pentagon was hit by a truck bomb or missile, how can it be that so many people saw the plane fly into the building?"
What do they say to the countless witnesses, who like James Robbins, have a simple response to these conspiracy theories about truck bombs and missiles hitting the Pentagon,
"I was there. I saw it. That is my entire rebuttal."
No matter how you spin it, what claims you make, or what your "evidence" that a missile or a truck bomb hit the Pentagon is supposed to be, it's all irrelevant compared to witness after witness after witness who say that they watched a plane fly into the Pentagon.
This is the problem the truthers have on every issue: the evidence against their conspiracy theories is so overwhelming that almost no one who is well informed about the issue could possibly believe them.
Now, we could get into some of the specific claims that the truthers make. For example, there's the claim that, "the fire in the WTC wasn't hot enough to melt steel." That was true initially (although it became hot enough after the buildings fell and the fires burned on under the rubble), but steel begins to lose its strength at 400 degrees Celsius and at 980 degrees Celsius, only a small fraction of its strength remains. The damage to the buildings' structural supports and the weakened steel caused by the fire brought down both the Twin Towers and World Trade Center Building 7. If you want to see that kind of point by point analysis, Popular Mechanics, which was mentioned above, has a whole book filled with it.
But, there's no point to doing that because this issue has been investigated over and over again -- and by sane, rational people, not truthers who think that all of our elections are fixed, that the Illuminati run the world, and that the US, Mexico, and Canada are going to merge into one giant North American Union next year.
The 9/11 attacks, or at least parts of those attacks, have been investigated by the 9/11 commission, the CIA, FBI, FAA, FEMA, The National Institute of Standards and Technology, the Senate Select Committee on Intelligence, the House Permanent Select Committee on Intelligence, Popular Mechanics, and countless mainstream newspapers -- among other sources.
None of these sources have concluded that there is any sort of grand conspiracy going on, that Bush was behind 9/11, that the Pentagon was hit with a truck bomb, that WTC 7 was brought down by a controlled demolition, etc.
So in other words, to believe in many of these kooky conspiracy theories, you have to believe that tens of thousands, maybe even hundreds of thousands of Republicans, Democrats, Independents, politicians, bureaucrats, journalists, FBI agents, and CIA agents all know about an incredibly complex, monstrous plot against the United States, and are keeping their lips sealed while Charlie Sheen, Rosie O'Donnell, and the fruit loops who think Bush is a puppet of the Freemasons have figured it all out.
For example, look at it like this: if you believe that a truck bomb hit the Pentagon and the government covered it up, you're in effect saying that you believe Bush, his cabinet, the FBI and/or CIA (who would have executed it), the eye witnesses, the rescue workers on the scene, people who work in the Pentagon, the newspapers that investigated it, Osama Bin Laden (who took credit for the attacks) and numerous other people all know that a truck bomb hit the Pentagon and are choosing to remain silent about it.
When I talk about looking at the big picture as opposed to conspiracy theorists focusing on tiny details, this is exactly what I'm talking about. Once you get out of the weeds where the crazies spend all their time looking at nearly irrelevant loose ends and start looking at everything that would have to fall into place to make this sort of X-Files scenario work, you can clearly see that it's impossible.
That's what people need to remember about 9/11: it has been investigated, in depth, by people from both parties, legitimate news organizations, and even Popular Mechanics, which is probably as close as you can possibly get to a non-political, non-partisan, unbiased organization in this instance and the conclusion is that there is no "there, there" to these conspiracies. That's why the only people who buy into them are the uninformed, the mentally unstable, and fools who don't understand that believing nothing that the government says is every bit as foolish as believing everything that the government says.