The combustion of hydrocarbons (e.g., fossil fuels) ALWAYS yields water vapor as a byproduct.
Under the proper atmospheric conditions, that water vapor can condense into fog or freeze into ice crystals.
That's what a con-trail is, ice crystals.
It's the same thing that causes car exhaust to be visible in cold weather. Water vapor -- the byproduct of combustion -- in the exhaust gasses is condensing into fog.
It's also why water sometimes drips from the tailpipe of cars left idling. The water vapor has condensed inside the exhaust pipe.
