Most of the people that believe this are the same conspiracy theorists that swear there was a second gunman, that we didn't land on the moon, and the "victims" of 9-11 are in the witness protection program because it was an elaborate plot to bring us to war. Oh, the second plane to hit the towers was also a specially modified missile plane painted in a UA scheme. These people also typically like to answer a question with another question.
Yeah... Chemtrails.