The 911Truth people have studied this problem for a so long time that I have not wanted to add my layman suggestions thinking that I cannot contribute anything. Yet, now I decided to propose a base-line demolition theory which has the merit that it agrees with the official theory in almost everything, except for being a demolition theory.
The official theory is that all three WTC buildings fell because fire in one floor in each building weakened steel so much that one floor collapsed. It led into collapse of all floors.
Let us assume that this theory is possible: if the steel pillars of one floor are weakened by fire, the whole building collapses. I have seen some calculations of WTC and it seems possible that the whole building would collapse. The falling speed of WTC7 presents a problem, but the supporters of the official theory try to explain it in some way. Let us grant all these explanations. Thus, I assume that if the steel pillars of one floor are weakened, the buildings fall just as was seen.
The base-line demolition theory is that the steel pillars of this one floor did not weaken because of plane crashes in WTC1 and 2 or fire from falling debris in WTC7. The alternative is that the pillars of these floors were weakened by high temperatures that were intentionally created for instance by covering each pillar with a sufficiently large thermite wrapping. The thermite wrappings were ignited at the same time. There were no loud bangs from cut charges, nor were these wrappings used as cut charges. They only needed to heat a part of the pillar to the weakening temperature in some suitable time that fits to the time difference between the plane crash and the collapse. If you do not like thermite, replace it with something else that can explain the high temperatures in Ground Zero long time after the event.
Compare these two theories. They have exact the same collapse mechanism, thus if you support the official theory, you cannot argue against this base-level demolition theory. No large demolition team needs to plan this collapse for a long time. Weakening steel, e.g. supports for electric lines, by fire is a basic military pioneer trick that every military knows. As long as all pillars weaken in more or less the same time the building collapses down more or less to its footprint.
What the official theory cannot explain is why something was burning in Ground Zero long time after. The base-line demolition theory explains it: when the pillars weakened there still was left quite much ignited thermite. It burned fast in Ground Zero and produced a very high temperature. As it was covered with rabble it took a long time to cool.
The official theory also cannot explain how a non-symmetric damage from a plane managed to weaken all pillars to cause a symmetric collapse. The base-line demolition theory explains it nicely: it was not any lucky coincidence that happened three times on that day. The wrappings were installed to burn at the same time.
The demolition theory explains why Silberstein said “pull” and why a fireman counted one, two, three”, and how the Israeli team of dancing Palestinians knew how to appear there with a video camera before the planes crashed.
Finally, if one floor was weakened with thermite, it explains why molten steel seems to be falling from the building: that is the place where the thermite currently was. It also explains why aluminium spectrum and iron spheres were present in the dust.
So, that is my simple base-line theory. I do not insist that this theory is the correct solution. For instance, you may argue that WTC7 fell too fast. But this theory is meant as a reply to supporters of the official theory. Nobody believing in the official theory can discard this base-line demolition theory as impossible because both are almost identical. Considering more than the collapse, the base-line demolition theory is clearly superior.
I could stop here, but will briefly comment the nuclear demolition theory that my valued friend Iris has supported. But no, I cannot support this theory. It is true that a camera was shaking before the collapse and it can well indicate an explosion. It is true that nuclear bombs were planned to be used for demolition. And it is true that there are bunker blasters and other mininukes and a mininuke can be spiced e.g. by a thermite wrapping.
But I have an objection to the theory that the buildings came down by an underground explosion of a large or small nuclear bomb. Ignoring radioactivity, which possible could have been less, there is this simple problem that if the ground falls down under a building or the central core, then the building starts to break into pieces in the down part. That is where the building hits the ground. Yet in the videos the buildings start to break in the levels where the planes hit (I am not saying that these planes were real, only that there are planes in the videos, the buildings do not break apart in the ground floor). Thus, I reject the nuclear demolition theory. Notice that such an idea that a shock wave destroys the building up to some level and the not destroyed upper part comes down as a piston is false: the whole building has mass and falls down, so it breaks at the bottom when it hits the ground.
There is also a theory that the whole building was wired for demolition, i.e., there were explosives or thermite on many floors. This is of course possible, but I think the base-line theory is simpler and as long as supporters of the official theory stick to their theory, one can grant them that it is not impossible and take the proposed collapse mechanism to the base-line demolition theory. The main strengths of the demolition theory are the high temperatures in Ground Zero and pre-knowledge. They are enough, the exact collapse mechanism is not essential for concluding that it was a demolition and who did it.
As for the no planes theory, I am quite open to it. Assuming that only one floor had burning material wrapped around pillars, it would be much of a chance to have the plane hit just that floor. The natural solution to this problem would be that the planes were added to the videos by computer graphics. There may or may not have been planes, but if they were, they probably hit some other floor. This means that there may have been planes and we still may be shown videos that are fabricated: the real plane did not hit the correct floor so they made a better video.