Here's why:

* Body Temperature: Your body maintains a core temperature of around 98.6°F (37°C). This is well above the freezing point of water (32°F or 0°C).

* Mucus: The lining of your lungs is coated in mucus, which helps to trap foreign particles and keep the airways moist. This mucus also helps to regulate the temperature and humidity within the lungs, preventing ice formation.

* Breathing: The air you breathe in is warmed and humidified by your body before it reaches your lungs. This process further reduces the possibility of ice formation.

While it's possible for ice crystals to form in the air you breathe in extremely cold environments, they won't form inside your lungs.