How many of the core columns were damaged at the level of the plane crashes in both buildings, Royston? And just for the sake of argument, let's say that the core columns at the level of the plane crashes were fatally damaged (there is also evidence that they were not, however), how would that effect the columns below the level of the plane crashes? What caused all of the rest of the core to fail? What is the mechanism that allowed this to happen? How did that work?