Nobody disputes that when you see an airplane high in the sky, you often see a white trail behind. But are these simply aircraft contrails or -- as increasing numbers of people believe -- are they the tell-tale signs that the government (or maybe a "shadow government") is spraying chemicals into the atmosphere as part of a covert program?

It won't be easy, but it's time we tried to make sense of one of the most intriguing conspiracy theories of recent times…