[PeteVS]

Gasoline molecules are basically composed of some crazy arrangement of Hydrogen and Carbon. (It's a "hydrocarbon" material.) When gasoline burns in the presence of Oxygen, the Hydrogen and Carbon separate and they both combine with the Oxygen, forming "oxides." There are two forms of oxides of Carbon- Carbon dioxide (two atoms of Oxygen with one atom of Carbon, and Carbon monoxide (one atom of Oxygen with one atom of Carbon. If the mixture of gasoline and Oxygen is incorrect, there might also be small of plain carbon, or black soot. The oxide of Hydrogen is Hydrogen dioxide (water.) Since the oxides produce large amounts of heat, the water exists in the combustion chamber as steam and if the engine and exhaust system are hot, it will exit the tailpipe as invisible steam. When the engine is shut off, all of the steam that's in the exhaust system will condense into water droplets as the metal cools off and lie in the pipes and muffler until the engine is restarted, and then the new exhaust will blow the old water, and soot if there is any, out the tailpipe.