Have you ever watched an airplane fly overhead, leaving behind trails of condensation? Most likely you have. And so have some conspiracy theorists, who believe that the emissions are part of a governmental "secret-spraying program" which harms the humans and the environments. This program is known by both "chemtrails" and "covert geoengineering." However, a study conducted at Carnegie Mellon asked 77 scientists about the authenticity of chemtrails, and 76 found no credible evidence. If anyone tries to convince you about chemtrails, remember that the theory is bunk.
Image Source: By Делфина (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons