system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is our body doing when we’re yawning and stretching ?
gpt
The truth is that no one really knows. The old explaination of needing more oxygen has been heavily debunked

The current leading theory is that it helps to cool the brain and stimulate bloodflow to the head, but the "why" isnt fully understood on that. The current theory is that it's your body trying to make itself more alert, and they think thats why it is contagious; its a social signal that you need to be more alert.