Have you ever found yourself starting to yawn just because you saw someone else do it? It's not your imagination: Yawns really are contagious. In fact, humans can feel the urge to yawn after seeing, hearing or even just thinking about someone else doing it.

Scientists have been trying for a while to figure out why yawns are so easy to "catch," but for now, they're not even sure why the yawns you let out when you're sleepy or bored happen. It's possible that a yawn (which is a big inhale and a stretch in your eardrums, followed by a large exhale) serves to cool down your brain or other parts of your body. It could also have something to do with regulating the amount of oxygen in your blood. It might even be a reflex that serves to keep your body alert when you're tired or distracted — something that our ancient primate ancestors evolved to keep them from zoning out and ignoring signs of predators.

That's one theory about why yawns are contagious: If they're meant to perk us up when we might be in danger, it's useful for them to spread quickly through a group. It's common for social animals such as humans to copy one another's behaviors to fit in, especially when that behavior is something that might be useful to their survival. That's why some research focuses on whether contagious yawning is related to empathy or our ability to understand and share what other people are feeling.

"Empathy can be defined by the ability to understand, share and be affected by the states or emotions of others. So, if seeing someone yawn makes you yawn in response, this action could be placed within a category of empathy," says Andrew C. Gallup, assistant professor of psychology at SUNY Polytechnic Institute in Utica, New York. Some studies have tried to see whether people and animals with more empathy are more likely to catch yawns. But Gallup, who has researched yawning for more than 10 years, says more study is needed to know for sure.

While yawns and their catchiness remain mysterious, we do know we're not alone in spreading them around. In addition to some other primates, scientists have seen evidence of parakeets and domesticated dogs catching yawns. Dogs actually catch yawns from humans, not other dogs, which suggests it might be something they picked up only after we started breeding them to be more human-friendly.

If we can figure out why these other animals share yawns with their friends, it might help us understand why we do it, too. And that could help us figure out why we tend to copy other reflexes.

"There are a number of other behaviors that are contagious, such as laughing and itching," Gallup says. "Similar to yawning, these responses can be reliably triggered by having participants watch videos of other people performing them."

Feltman is an editor at Popular Science magazine.