You know, those loons that cannot believe that 9/11 was a terrorist attack and that instead our own government attacked America on 9/11. No, it isn't an appeal to eyewitnesses, of whom there are many. It isn't an appeal to science, which overwhelmingly demonstrates that the official version is correct. Rather, it is a simple appeal to logic.
[T]o 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.
So much like the claim that the moon landings all took place in a soundstage, simple reality cannot sustain the conspiracy claims. After all, given the inability of small groups of people to keep small secrets, there is no way that a large group of people could possibly keep "the truth" hidden if 9/11 was a government conspiracy.