What is the best way to explain to a ''Chemtrail'' conspiracy theorist what those trails in the sky actually are?

Posted by Cuppagreenteacafe@reddit | aviation | View on Reddit | 202 comments

English is not my first language, but one of my friends, who is from the United States, sends me photos of airplane trails in the sky telling me that the planes are ''spraying'' stuff. I know that isn't true and the trails work in a similar way to clouds, I am just wondering what is the best and most simple way of explaining it to this person ! Thank you !