One of the most consistently used facts by 9/11 conspiracy theorists is that the grade of steel used in the World Trade Center simply couldn't melt at the temperatures fires created when jetliners slammed into the structures. And they're right.
Steel's melting point is 2,750 degrees Fahrenheit (1510 Celsius). Yet jet fuel only burns between 800 and 1500 degrees Fahrenheit (426.7 and 815.5 Celsius) [source: Popular Mechanics]. So what happened on that September morning that caused the towers to collapse?
The steel bent.
Even though the girders that comprised the twin towers wouldn't turn to molten steel in the jet fuel fire, they would certainly have weakened in the heat. In fact, one estimate says that they would have lost half of their strength at 1,100 degrees Fahrenheit (593.3 Celsius) [source: Popular Mechanics]. It's important to also note that other items would have caught on fire in the buildings in addition to the jet fuel, and could have contributed to higher burning temperatures.
But according to a multi-year study carried out by the National Institute of Standards and Technology (NIST) even these fires combined with the impact of the planes would not have been enough to bring the buildings down. Their report concluded that the real culprit was the fact that fireproofing materials were dislodged during the impact and this in turn exposed vital structural components to the heat. As these components lost strength, the floors began to sag, pulling the weakened columns inward and leading to a chain reaction that had floor collapsing upon floor until the buildings no longer stood [source: NIST].