why do we yawn?

While on holiday, a combination of jet lag and overstimulation correlated with my seven-year-old yawning more than usual, and wondering why. Seems that others also stay up late, perhaps yawning, considering why we yawn.

When do we yawn?

We yawn before we are born. Maybe it's boring to sit in a womb all day. Most vertebrates yawn, though I don't know how they tell in fish. Our rate of yawning does not seem to depend on the amount of oxygen or carbon dioxide in the surroundings. We are, however, more likely to yawn when it's cooler, and more likely in winter.
You may have also noticed that seeing someone else yawn can make you yawn. This is called contagious yawning. Just reading about yawning can make you yawn (see if you do before finishing this post). Contagious yawning is more likely to happen when we haven't been using our brain much. We aren't affected by contagious yawning until we are more than about five years old. And this form of yawning is only known in primates and perhaps dogs. Some people won an Ig Nobel prize for showing that it does not work in tortoises. Those of us with greater empathy are more prone to contagious yawning. And we are more likely to catch a yawn from someone with whom we are close. Those of us with autism or schizophrenia do not show contagious yawning.

What happens when we yawn?

You've probably noticed that when you start a yawn, it's really hard to stop. It's an involuntary action. The stretching component has a great term — pandiculation.
In rats, yawning cools the brain, something about the effect of a countercurrent in air flow. I wonder if it has anything to do with getting a brain freeze.
Functional magnetic resonance imaging (fMRI) shows that when we look at facial expressions in people, mirror neurons activate, indicating a kind of mental reenactment. Yet when we see somebody yawn, these neurons do NOT fire.