There are some 911 conspiracy facts that everyone should know if they're interested in September 11th conspiracies. 9/11 conspiracy theories started as a way to make sense of something that had never happened before - the organized hijacking and crashing of four jetliners on US soil. Almost immediately, the nascent internet conspiracy movement seized on things that "didn't make sense" and "didn't add up." They didn't believe it was plausible for hijackers to take control of four planes with box cutters; in their minds, there had to be more to it than that.
Since then, September 11th conspiracy theories have revolved around two different central themes: that the government either made the attacks happen or allowed the attacks to happen. One says that the buildings were rigged with explosives and destroyed, using faked hijackings as a cover. The other says that the government didn't actively plan the attacks, but knew they were about to happen, and let them unfold without taking action to stop them. Other elements of possible 9/11 conspiracies include a missile fired at the Pentagon, Flight 93 being shot down on purpose, wealthy individuals profiting off the attacks, and evidence of the hijackings being faked.
In the years since the attacks, no 9/11 conspiracy theory on this list has been proven. They've mostly been debunked with facts, and those that haven't generally didn't boast enough evidence to debunk in the first place. The 9/11 truth movement, which exploded during the Bush administration, has mostly faded away, with hardcore truthers moving on to other conspiracy theories, leaving only these discredited theories behind.
"The Government Made It Happen"
Conspiracy theories that the 9/11 attacks were an inside job pulled off by the US government started on the internet hours after the planes made impact. Since then, rumors have run rampant that elements in the government planned the attack, arranged for the hijackings, destroyed the buildings through "controlled demolition," and did it all to make money and/or create an excuse to invade Iraq.
After George W. Bush left office, these rumors started to die down, but they still have a considerable presence - despite absolutely no credible evidence ever being found that corroborates the idea the government made the attacks happen on purpose. If it is true, then the conspiracy would have involved thousands of people scattered across numerous branches of government and in multiple locations rigging the Twin Towers with explosives without anyone finding out, hiring hijackers (or actors to play hijackers who were willing to die), faking untold quantities of evidence, enlisting countless experts to play along, and never letting a word of it out into the public.
This is a conspiracy so sprawling it strains the very basics of logic.
"The Government Allowed It to Happen"
An alternative version of the attacks being an inside job held by some conspiracy theorists is that the government didn't actively plan the attacks, but knew they were going to happen and allowed them to. This is a version of the "Pearl Harbor advance knowledge" hypothesis - and like that conspiracy, this one has little to corroborate it other than certain coincidences and failures of intelligence.
The most damning piece of evidence is that the Bush administration was given a memo several weeks before the attack claiming that Osama Bin Laden was planning an attack in the United States, but even that document (which was declassified in 2004) had no concrete information on where or when it would take place - only that it might.
"Jet Fuel Can't Melt Steel Beams"
A conspiracy claim that's turned into an internet meme, this holds that the temperature of burning jet fuel doesn't get hot enough to melt steel - meaning the towers had to be brought down by an explosion.
While it's true that jet fuel burns at between 800° and 1500°F, and steel melts at 2750°F, this meme doesn't take into account that the beams didn't have to melt - they just had to soften and lose enough of their strength to be unable to hold up the floors above them. Based on the way the WTC fell down in a sort of straight pancake, it's reasonable to conclude that this is what happened.
"WTC 7 Was Destroyed by a Controlled Demolition"
People watching the aftermath of the attacks were shocked when World Trade Center Building 7 collapsed, despite not being hit by either of the planes. A conspiracy theory started almost immediately that the building had been wired with charges and destroyed on the order of building owner Larry Silverstein, who ordered it "pulled."
While WTC 7 wasn't hit by a plane, it was hit by a large amount of debris, including the perimeter columns of one of the Twin Towers, which tore a massive gash in the side of the structure. Building 7 also had a large amount of burning jet fuel hit it, which started fires that were never put out due to failures in the sprinkler system because of water lines broken in the attack. Finally, the infamous "pull it" quote doesn't relate to some kind of demolition charge, but to the firefighting team trying to put the blaze out. The technical term for demolishing a building isn't "pull," but "shoot" or "blow."