[ZebraRampage]

The WTC was strong enough to stand up even with those planes crashing into them. There was one problem though. The supports to every floor(I can't remember what they're called) didn't have fire proof on them(I'm not entirely sure about this), and when the planes crashed they were carrying so much fuel that the fire was so hot and intense that it destroyed the supports. Without that support the WTC couldn't hold any part of the building above the damaged area. Once it was too much to hold, the damaged area of the building collapsed and the part on top of it was so heavy that it kept plowing the rest of hte building down with it. This is also why it collapsed straight down.

I saw this on the discovery channel I believe, so I don't have any links to back this up, but I know it's true because I saw the documentary about it.