Why is yawning contagious?

There is no doubt that yawning is contagious. I mean, imagine someone with their mouth wide open, squinting eyes, taking a long breath in, then a short one out. Are you yawning yet? Almost anything that reminds us of yawning, like reading about it, seeing a video of it, will cause us to yawn. Just thinking about yawning can be enough. It’s actually quite hard to write about yawning because you start yawning all the time. During the research and writing of this post I yawned 34 times. I counted. Ok, make that 35 times.