...or the "jetfuel can't melt steel beams" kind.
I never understood why that was supposed to be a relevant point in the first place. The towers didn't melt, they were brought down by a rather basic and mundane application of Fudd's First Law of Opposition.
In the case of 9/11, the loss of structural integrity came about from the steel beams being softened by the fire to a sufficient extent that the upper floors could no longer support their own weight, causing that floor to slam into the floor below it, which lost integrity and both floors slammed down on the next one, repeating through the entire structure. The Twin Towers were (deliberately) designed to "pancake" in in this manner in an effort to reduce collateral damage. To the uninformed (those without significant knowledge of structural engineering, material science, and/or demolitions), this superficially resembles the sort of collapse that a controlled demolition produces.
A small number of people seized on this (in the first few days after the attacks, when very little information on what happened was available) to insist that this WAS a controlled demolition, usually in concert with photos (either very selectively selected or doctored) that showed the damage to the Pentagon but not the aircraft wreckage, claiming that no such wreckage exists. These people quickly gained a huge following, because people always look for a comfortable explanation after a tragedy, and it just seemed so implausible that such an attack could occur and be missed. When the official reports on the method of collapse appeared, the "jet fuel can't melt steel beams" line was used to attack the explanation.
In general, a conspiracy theory is much like a house built upon a platform of toothpicks - any individual support is flimsy and very easy to break, but there's so
many such supports that loss of one, or several, doesn't weaken the house at all, and they can be replaced faster than you can tear them out.