As you watch film of the towers coming down, there is an impression that the lower parts don't go anywhere, but rather seem to just fold up like thin paper.
Have you ever seen a greeting card which, as you open it, a 3D form spreads open? Then as you close it again, the 3D "solid" object just folds flat.
That was the illusion; as the towers collapsed, they appeared to just fold down on themselves as though they were paper "folding under".
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