You've probably noticed those wispy white trails that sometimes appear across the daytime sky. NASA's website says that these are man-made clouds, formed when water vapor condenses around tiny particles of pollution from the exhaust of jet aircraft, and forms streams of short-lived ice crystals that eventually vanish as they return to a vapor state.
But to conspiracy theorists, that explanation seems a little, too, ah, convenient. They maintain that the contrails are actually "chemtrails," which are deliberately released from aircraft as part of a sinister, secret plot to alter the weather. As an article on the website Illuminati Agenda explains, one suspicion is that the man-made clouds help to concentrate energy weapons that manipulate electricity in the ionosphere, a part of the atmosphere. Another possibility is that the clouds may be part of some larger plot to block sunlight and/or poison the soil to reduce farm yields, forcing humanity to become dependent upon genetically modified food crops that, of course, the Illuminati control.
Most likely, the anonymous author argues, all of these things are happening at once: "It makes obvious sense to the illuminati to make use of all potential avenues instead of just one as it allows them to bombard us from numerous angles which gives us little room for maneuver." Obviously.