Here's why:

* Nitrogen's boiling point: Nitrogen's boiling point is -195.8°C (-320.4°F). This means that at temperatures below this point, it exists as a liquid. Above this point, it's a gas.

* Nitrogen's freezing point: Nitrogen's freezing point is -210°C (-346°F). So, it needs to be even colder than 0°C to become a solid.

Therefore, at 0°C, nitrogen is well above its boiling point and exists in its gaseous state.