system
Given the following post, respond as if you're commenting on Reddit:
human
Why is breath cold when you blow in an 'o' but warm when you breathe out with a wide mouth?
gpt
Take a sheet of paper, and roll it into a tube.  Now, blow through the tube with an 'o' - shaped mouth, and put your hand on the other side.  Once you push the ambient air out of the tube, you will feel your warm breath.

When you blow a narrow stream of air out of your mouth, ambient air is dragged along with it.  This mixture of breath and ambient air feels cold.  When you breath with a wide open mouth, the air stream is much wider (and slower) and doesn't grab as much ambient air.

Blowing through a tube prevents the ambient air from entering the stream.