Numerous theories abound as to what a happened on September 11, 2001 when two airplanes struck two of the buildings of the World Trade Center. The buildings collapsed within minutes if not seconds. Later building number seven collapsed onto its own footprint. It was at some remove from the other buildings and it was not struck.
If one believes the government story, the fuel onboard the planes crashed into the towers created a fire so hot and intense it caused the buildings to free-tail onto their own foundations.
Scientist have noted that jet fuel does not burn hot enough to melt steel. I am no scientist, but I have done a little welding. Even with a hot enough flame, it takes two or three minutes to heat a thin piece of metal to get it hot enough to become liquid and lose its structural integrity.