Steel melts at 2,777 °F, but jet fuel burns at only 1,517 °F. Steel loses 50% of its strength at 1,200 °F; 90,000 L of jet fuel ignited other combustible materials such as rugs, curtains, furniture and paper, which continued burning after the jet fuel was exhausted, raising temperatures above 1,400 °F and spreading the inferno throughout each building. Temperature differentials of hundreds of degrees across single steel horizontal trusses caused them to sag--straining and then breaking the angle clips that held the beams to the vertical columns. Once one truss failed, others followed. When one floor collapsed onto the next floor below, that floor subsequently gave way, creating a pancaking effect that triggered each 500,000-ton structure to crumble. Conspiricists argue that the buildings should have fallen over on their sides, but with 95% of each building consisting of air, they could only have collapsed straight down.