William Saletan of Slate summed it up well:
To believe that the U.S. government planned or deliberately allowed the 9/11 attacks, you’d have to posit that President Bush intentionally sacrificed 3,000 Americans. To believe that explosives, not planes, brought down the buildings, you’d have to imagine an operation large enough to plant the devices without anyone getting caught. To insist that the truth remains hidden, you’d have to assume that everyone who has reviewed the attacks and the events leading up to them—the CIA, the Justice Department, the Federal Aviation Administration, the North American Aerospace Defense Command, the Federal Emergency Management Agency, scientific organizations, peer-reviewed journals, news organizations, the airlines, and local law enforcement agencies in three states—was incompetent, deceived, or part of the cover-up.
This thought process has been analyzed by serious minds over and over. But it seems like mockery has been the most effective tool in combating purveyors of pseudoscience, and the folks over at CRACKED demonstrate this well with their new video, The Truth Behind Every Internet Conspiracy Theory.