system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 When you exhale, why is your breath cool when you purse your lips but warn when you open your mouth.
gpt
The two explanations currently in this thread are wrong. 

The reason your breath feels cooler when you purse your lips is because you're not really feeling your breath, you're feeling the ambient temperature in the room. 

According to the venturi effect, when a fluid (air in this case) passes through a constriction, it's velocity increases and it's pressure decreases. Because the pressure of your breath decreases when you purse your lips, ambient air rushes in to equalize the pressure. Because the air you're blowing is now made partially of room temperature air, the average temperature decreases.

 As an experiment, you can try blowing on hand somewhere where the ambient temperature is above your body temperature, and you'll feel that the temperature actually rises.