Fractional distillation is the most common method for separating the gases in air. This method is used because the gases in air have a difference in boiling points of less than 25 K, making them suitable for fractional distillation.

Here's how fractional distillation works:

1. Liquefy the gases: Air is converted into liquid air by increasing the pressure and decreasing the temperature. 

2. Separate carbon dioxide and water vapour: As the temperature drops below 0°C, carbon dioxide separates as dry ice and water vapor separates as ice. 

3. Pass the liquid air through a column: The liquid air is then passed through a fractional distillation column, where the gases are separated based on their boiling points.