The air you breathe out has different amounts of oxygen, nitrogen, carbon dioxide and a few other substances in it than the air you breathed in. Your lungs absorb a lot of the oxygen, for example,...
It is usually warmer, but not always. The temperature of the interior of your body is about 98-99 degrees Fahrenheit. So when you breathe in air that's cooler than that, the air is warmed by your...
Instinct. It's automatic. The doctor slaps you on the bottom and you start to breathe! So you don't even have to think about it. (You are human, aren't you?)