When you meet up with 911 truthers, I find that ridicule works best. They simply cannot conceive that 1500 degree flames sill reduce the strength of steel by 80%.
Building 7 - 47 story skyscraper - that was home to all the alphabet security agencies fell because parts of the neighboring towers fell on it. Then fell at free fall speeds. Funny as it was announced live on the news that it collapsed while it could still be seen standing in the background on 9/11. Ridicule and mocking and ad hominum attacks are definitely preferable to reason.