A crazy commenter, Stonehillady, got me thinking about how the World Trade Center towers fell. In turn, I found a great site that explains it very well, while pointing out the flaws in the half-baked conspiracy theories.
Here's one: how many pounds of thermite would be needed to bring down the Twin Towers? I read it somewhere (can't find it now), and the load would have taken something like nine trucks full of thermite. Then the charges would have had to be placed and wired, and who knows what else. And it was all set up to coincide with terrorists flying planes into the towers. Amazing planning that must have been! Literally incredible and unbelievable.
So, if you don't believe the thermite hypothesis (hint: don't believe it), here is a fairly thorough explanation of why the towers collapsed. I can't give a particularly short version of the explanation, except to say it was a combination of the construction, impact of the plane, fire, and buckling of the supports under those conditions. Read the whole thing if you're interested. I learned a lot.