OK, I lied, I'm going to post one more, but only because this one's actually really easy.
Like I keep pointing out, we're only used to the knock-a-glass-over type of physics. The center of gravity of a glass of water is very high, about 1/3 to 1/2 way up the glass usually, and the support base in and of itself is very small.
In order for an object to tip over, the center of gravity has to leave the support base - in other words, the object has to tip a certain distance to reach that 'breaking point' where it's going to tip over. A glass of water is even worse for this because once it starts to tip, the water moves, causing the center of gravity to shift further over the edge. Once the center of gravity leaves the support base, it will fall sideways.
With a building like the World Trade Center, the center of gravity is MUCH, MUCH lower by design, and the support base is ENORMOUS.
I found a resource explaining this phenomenon and in the process, that same resource even talks about the steel failing!
http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html Good read!
Here's the pertinent information about why the building pancaked instead of toppling:
Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.1 It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a 500,000 t structure has too much inertia to fall in any direction other than nearly straight down.
That's why the WTC collapsed. Straight down. Thud.
Planes WILL do that. Again, you have to consider that the physics involved here are VERY different from what you're used to seeing, thus the events of a skyscraper collapsing will NOT match up with every day occurrences your mind is used to working with.
edit: note also that the impact and subsequent collapse did send debris flying all over, which could very well have hit the WTC6 building that "mysteriously" collapsed as well. Nevermind the fact that a good million tonnes or so of concrete and steel just came crashing down, which surely also caused some shocks across the foundations.
edit 2: for some fun, if you want to see just what a low center of gravity and larger support base can do -
http://www.timesonline.co.uk/tol/life_and_style/court_and_social/article4790465.ece Check out that double decker bus tilt test! There are even sandbags in the top level to represent the weight of people in it. Look how far the bus can tilt before tipping over! Same concept with the World Trade Center, only the WTC is thousands of times larger and more massive even than said double decker bus.
