system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5:What exactly IS a yawn?
gpt
A yawn is a coordinated movement of the thoracic muscles in the chest, diaphragm, larynx in the throat, and palate in the mouth. By yawning, we help distribute surfactant (wetting agent) to coat the alveoli (tiny air sacs) in the lungs. Generally speaking, we cannot yawn on command. It is theorized that yawning is a semi-voluntary action and partly a reflex controlled by neurotransmitters in the hypothalamus of the brain.