Just like the CPU in your computer, the human brain has an optimal temperature where it runs best. But unlike a computer's CPU, there's no built-in fan to chill the brain when it starts to run hot. Which is why researchers now believe that yawning is actually the body's physiological way of keeping the brain nice and cool.
Traditionally, it was believed that yawning was how the body increased its oxygen intake when someone was sleepy, to help wake them up—but this is a more interesting conclusion. The act of yawning increases your heart rate, blood flow, and uses muscles in the face that are all connected to keeping the brain cooled. It also turns out that exhaustion and sleep deprivation can contribute to a rise in the brain's temperature, which is why we tend to yawn more often when we're tired.
But why do we yawn when others do? It's believed that that phenomenon is tied to our ability to empathize with others. Which is why it's harder to get kids below the age of five to yawn when you do, because their empathetic abilities have yet to fully develop. And it also explains why people with autism are less likely to start yawning when they watch someone else do it.
So, how many times did you 'cool your brain' while reading this post or watching that video?