As I watched the towers collapse, one after the other, I wondered why they pancaked rather than falling over (or just standing until the fires burned themselves out.) Thinking later and considering the evidence - each tower probably weighed something north of 500,000 tons and 20%+ of that was uphill from the points of impact. The fires involved 4-5 floors, say 50 feet, and when the structural steel heated until it lost most of its tensile strength, 100,000+ tons dropped that ~50 feet with little resistance. The impact of that huge mass hitting the undamaged floors below - nothing was going to even slow it down.