so why did the twin towers collapse?

is a huge 727 airplane on fire, inside of the building, loaded with jet fuel....stressing the steel joints and heating up the steel support beams to melting point thus causing stress on lower steel support beans that are melting thus causing overwheling stress on lower floors steel support beams that are melting from heat from fire from a fully fuel loaded and already burning commericial plane that finally the wight bearing melting beam can no longer tolerate the weight from above floors that have lost all possible support abilities..