On a sub-atomic level I think it's accurate to say that everything is mostly air. Atoms have a very dense nucleus that's orbited by electrons equal parts matter and waves, leaving "space" as the primary component of each atom.
The WTC Towers all had structural steel skeletons extending from Manhattan's bedrock to the top most floor in each building.
In the case of the shortest tower, WTC7, which was NOT struck by an airplane and collapsed at 5:20PM on 9/11/2001, 40,000 tons of structural steel extending 47 floors above ground level wouldn't seem like the path of least resistance.
"The overall building mass fell uniformly and with
almost perfect symmetry through what should
have been the path of greatest resistance – 40,000
tons of structural steel. This requires a precisely
timed patterned removal of critical columns –
which office fires, a gradual organic process, is not capable of.
"Only a carefully engineered implosion
could cause this 47-story building to collapse in on itself – and land mostly within its own footprint. After all, demolition
companies are paid large sums to accomplish this feat, and only a few can do it with tall buildings.
"Also, the destruction was complete. The building had been built especially strong so that alternate floors could be removed in case a tenant needed an extra tall space.
"Yet its forty-seven stories were reduced, in less than seven seconds, to about four stories of debris – like a house of cards – with the virtually complete dismemberment of the steel skeleton, including both braced and welded moment-resisting (bend-resistant) frames."
Republic Magazine Issue 16 P. 23