A kid threw a rock at it and it tumbled to the ground at free-fall speed. The conspiracy nuts claim it was demolished, but clearly the speed & angle at which the rock hit the building was enough to send a minor vibration to the inner steel columns, causing them to very briefly lose stability just long enough slightly shift the weight of the building and buckle the support structure resulting in the entire skyscraper falling to the ground in seconds. Easily explainable. A guy also farted and blew a hole in the Pentagon.