system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do vehicles "smoke" more from their exhaust pipes when they first start up, and then when the engine has warmed up, stops?
gpt
The 'smoke' you actually see is water vapour, the product of combustion of hydrocarbons and oxygen into carbon dioxide and water. (CxHx + O2 -> CO2 + H2O)

When the engine and exhaust gas is cold, or the weather is cold, this condenses much quicker in the air to give a denser "fog". When the engine is running at temperature and the whole exhaust system has heated up, the water remains as vapour, rather than condensation, so the exhaust gas is largely invisible.

There are of course other things in exhaust gasses since combustion isn't perfect. Diesel fumes, in particular, really do contain sooty particulates.When a diesel runs rich (youtube videos of diesel tractor pulls!) they really so make a tonne of smoke. Lots of effort is being made ot capture these particulates, since they are very harmful to health if inhaled.