I have been involved in a running comment battle and got into a discussion of the South Tower tipping. I came up with "The-Beer-Can" example which I thought I would share:

http://www.daytondailynews.com/news/dayton-news/were-twin-towers-downed-by-demolitions-group-thinks-so-906317.html

"The Beer-Can" Example (part 1 of 3):

He said: "... Well, see, yeah it did hit something - the lower intact structure!"

We are talking about the intact structure of the top 30 floors of the South Tower tilting 22 degrees. I said it should have toppled-over as an intact piece to be found crushed below.

You said it crushed everything below because, as it hit every floor, it destroyed that floor and it kept crushing. For this to happen, it must be completely disconnected from the structure below.

"The Beer-Can" Example (part 2 of 3):

To simulate this yourself, take two beer cans. Stack one on top of the other (i.e. make a two piece tower) and hold it out about an arms length away. If you don't shake, it will remain balanced. With your other hand, tilt the top beer-can 22 degrees. Like YOUR version of the tower story the top half is disconnected from the bottom half (or it couldn't fall because it would still be supported). Release the top beer-can and watch it fall to the side.

"The Beer-Can" Example (part 3 of 3):

If, when you let go of the top beer-can, you also start moving the second can downward, the top beer-can will continue to tip over and fall. Only if you drop both beer-cans at the same instant, so they are both in 'free fall,' will they remain in the same relative position (until the floor).

The only way for the lower beer-can to 'free fall' is if all the supports below it are destroyed. They had just been there.

Explosive demolition could do that!