Did you know why we yawn?

The most popular, but likely wrong, theory is that yawning increases levels of oxygen to the brain. A more plausible theory: Yawning cools the brain down. A study conducted on mice found that as mice sucked in air, their jaws stretched—this increases blood flow to the brain. This combination of cooler air intake when yawning mixed with the blood flowing to the brain is thought to eventually cool the brain down.