People who believe the theory have speculated about what purpose chemtrails may serve. Some say it is to change the weather or stop global warming. Others say that it is to control the population by spreading chemicals that are harmful to the human reproductive system. Some more recent versions of the theory say that chemtrails are being spread by companies that make genetically-modified (GMO) crops. The chemtrails supposedly contain herbicides that will only kill normal (organic) crops and will not affect the genetically-modified crops.