Thermite, and if you dont like this one, go get some fine steel wool and use a hydrocarbon fuel source, (like a bic lighter on high flame) and burn it. You might have to blow on it a bit to get it going good and voila, thermite. So in the WTC you have aluminum clad steel girders, an aluminum thing called an airplane hitting them at well above the sped the buildings were designed to withstand, thousands of liters of hydrocarbon up in flames, and a good breeze blowing in the hole left by that aircraft and voila, thermite, molten metal, pancaked buildings.