re: Explain what caused the towers to collspse.

Can Jet Fuel Melt Steel Beams?
"Given adequate oxygen, certainly. From here, for instance, you can get an approximate maximum flame temperature for kerosene burning in air, and a higher concentration of oxygen will increase the temperature. At 3800 F, this is about 1000 F above the melting point of steel, so melting steel with jet fuel (kerosene) is entirely possible.
Of course, "Common sense suggests that steel beams should not yield" suggests that you've been visiting 9-11 conspiracy sites, and it's important that you realize that melting beams is not remotely required to bring down a building. All you need to do is a) destroy some of the existing beams in order to increase the load on the survivors, and b) heat some of the survivors enough to reduce their strength to the point that they can no longer support their load. When some start to deform, this will throw extra load on the others, and a chain reaction of failure is certainly possible. And steel will certainly lose strength at high temperatures - typically about 50% at 1000 F, and 75% at 1100 F." LINK

How well do you expect the structural steel to continue supporting the structure when it has lost 80% of it's original strength when it is heated to 1200 degrees F? It doesn't have to "melt" to fail.
Although paper ignites at around 480 degrees Fahrenheit, it gets far hotter once it’s burning. The temperature at the center of a paper fire is 1,500 degrees Fahrenheit, give or take a couple hundred. The tips of the flames themselves are usually between 600 and 800 degrees.
Common building materials will burn at temperatures in excess of 3500 degrees when provided with adequate supplies of air and the adiabatic flame temperature of jet fuel is quoted at in excess of 4000 degrees.
LINK