Some stripes left in the sky by military aircraft may have a nefarious purpose, according to conspiracy theorists who speculate that the U.S. or other governments are releasing harmful chemicals from on high. Very similar in appearance to, but somehow to be distinguished from, contrails, which are simply harmless water condensation trails, chemtrails are supposed to be laced with various substances which, adherents of this theory believe, are used for secret government schemes, such as mass-vaccination against anthrax, blocking the sun's rays to prevent global warming, weather control, or weakening the populace to make them unable to fight the advent of a New World Order (q.v.). The U.S. Air Force dismisses these claims as misinformed misinterpretation of normal condensation trails, perhaps fed by a 1996 paper written at its Air University entitled Weather as a Force Multiplier: Owning the Weather in 2025. One believer in the conspiracy is the singer Prince, who recounted during an interview with PBS host Tavis Smiley his observation that chemtrails were linked with an increase in the number of fights in his neighborhood.