I figure that you guys must know fluid dynamics better than most physicists so here's my question: Open your mouth wide and exhale and it's hot air you feel. Now pucker your lips and exhale and it feels cold (unless you hold your hand right up close to your lips). Is this an example of the hot air from your lungs cooling because it is compressed by passing through a small opening or an example of air entrainment? Air entrainment is when a small amount of rapidly moving air draws additional air along with it. It would then be the additional air (at a lower temp. than that in your mouth) that makes the air feel cool.