breathing

It's not actually smoke but a steam which is produced whenever a warm environment collide with a cool environment. Our body or our core temperature is warm than the outside temperature and so when we breathe specially in a cold environment then it produces a steam or moisture like clouds.

It's not smoke, really, it's steam. It is the combination of your hot, moist breath coming in contact with the cold air, the same thing that happens when you take your hot coffee out into the cold, or boil water to make pasta. Or when the car windows fog up because it is warmer inside than it is outside (I have actually seen car windows fog up in the opposite direction - when the air conditioner was making it so cold inside that the hot air outside made condensation on the outside of the car. Only once.).

I don't know why exactly. But I'd assume it's freezing the moisture in your breath? & or changing it's molecular structure some how. Like I have a table top hot air humidifier, I put in cold water and out comes a very fine steam. I'm sure it's on a similar principal. Interesting question though.