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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, . As far back as 400 B.C., Hippocrates thought yawning removed bad air from the lungs before a fever. In the 17th and 18th century, doctors believed yawning increased oxygen in the blood, blood pressure, heart rate and blood flow itself. More recently, consensus moved toward the idea that , so when ambient conditions and temperature of the brain itself increase, yawning episodes increase.

Despite all these theories, the truth is that scientists do not know the true biological function of a yawn.

What we do know is that yawning occurs in just about every species. It happens . It can be in some species. Yawning can occur during times of social conflict and stress, something researchers call .

And that wide-open mouth , especially in social species such as , , , and .

Watching someone yawn – heck, even reading about yawns – can lead you to yawn yourself. Why?

Research on humans tell us that people who are more empathetic tend to be more susceptible to contagious yawning. When you see someone else yawn, the are activated.

Is yawning contagious for dogs, too? In 2011, U.K. biologists tested for contagious yawning . Although 5 of the 19 dogs they studied did yawn in response to an unfamiliar person's yawn, the researchers couldn't prove the yawns were contagious.

In 2013, cognitive and behavioral scientists at the University of Tokyo once again tested contagious yawning in canines while controlling for stress. This time the researchers found that dogs were more likely to . They concluded that dogs can "catch" a yawn from humans and that yawning is a social rather than a stress-based behavior.

In 2014, University of Nebraska psychologists looked at contagious yawning in shelter dogs. They found that some dogs that yawned when exposed to human yawning had elevated cortisol levels – a proxy for stress. Levels of the cortisol stress hormone did not rise in dogs that didn't yawn in response to a human yawn. This finding suggests and others do not. More research is needed to evaluate this aspect of the human-dog relationship.

So the jury's still out on the true why of yawning. But when it comes to inter-species yawning, you can collect your own anecdotal data. Try an experiment at home: Yawn and see if your pet yawns back.

This article was originally published on . Read the .