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Every September, apparently, we have to demolish the same conspiracy theory again.No, the World Trade Center did not need to be secretly blown up by the US government to create an excuse to invade Afghanistan.And no, "jet fuel can't melt steel beams" is not the devastating engineering argument some people still seem to think it is.Steel does not have to melt to fail.Modern skyscrapers are structural systems designed to redistribute loads. On 9/11, aircraft travelling at high speed severed and damaged structural columns, damaged floors and stripped fireproofing from parts of the steel structure.And yet the towers remained standing after the impacts.That is important.It means the remaining structure initially did exactly what a redundant structure is supposed to do: it redistributed the loads.Then large fires burned across several floors. Heated steel progressively loses strength and stiffness. Floors and structural members deform. Loads move elsewhere. Columns that were already damaged become increasingly overloaded.Eventually, part of the structure can no longer support the load above it.At that point gravity takes over.Once tens of thousands of tonnes of the upper building begin moving downward, the floors below are no longer carrying a static building. They are being hit by a huge moving mass. The loads involved become vastly greater than those for which an intact floor was designed.None of this requires steel to melt.None of it requires thermite.None of it requires explosives secretly installed throughout occupied skyscrapers without anyone noticing.And consider what the alternative explanation actually requires.You need a US government conspiracy capable of secretly preparing two enormous occupied office towers for controlled demolition, coordinating this with four hijacked aircraft, involving an unknown number of engineers, demolition specialists, intelligence officers and officials — and then keeping convincing physical evidence, documentation and credible participants from emerging for 25 years.All to obtain something great powers have historically demonstrated they are perfectly capable of obtaining without secretly demolishing part of their own largest city: a justification for military action.Extraordinary claims require evidence proportional to the claim.A complicated event is not evidence of a conspiracy simply because most of us don't understand structural engineering."I don't understand how that happened" is a perfectly reasonable starting point.Therefore explosives is not.And if rabbit hole dwellers tell you otherwise, remember to breathe.The air is not a conspiracy.Yet.Oh, no evidence of any missle was found at the Pentagon. There were Boeing 757-223 parts scattered at the crash site.