A version of this article originally appeared on The Conversation.

You know the feeling. It’s impossible to resist. You just need to yawn.

A yawn consists of an extended gaping of the mouth followed by a more rapid closure. In mammals and birds, a long intake of breath and shorter exhale follows the gaping of the mouth, but in other species such as fish, amphibians and snakes there is no intake of breath.

But what’s behind a yawn, why does it occur?

In the past, people have ha