The Vertical Collapse of Building 7
Building 7 collapsed in a nearly perfectly vertical motion at near the rate of free-fall. The first sign of the collapse is the falling of the penthouse, immediately followed by the falling of the whole facade, as seen from either the north and south. The middle of the building's north wall fell slightly faster than its edges.
The straight-down fall of the building's top can be clearly seen in photographs and videos. In the pair of images above, note that the position of Building 7's right wall remains in almost exactly the same horizontal position despite having fallen over 100 feet.
Dust engulfed the building when the roof had fallen to within about 15 stories of street level. The nature of the collapse past that point can be inferred by the shape of the rubble pile and the fact that it was covered by the remains of the exterior walls. This means that, not only did the building continue to fall vertically until the end, the outer walls were pulled inward so that they fell on top of the rubble pile.
In short, Building 7 imploded. Buildings are not designed to implode. They are designed to remain standing. To achieve a precisely vertical collapse, in which the remains of the building fall inward, is the objective of controlled demolition.