Yawn is that involuntary action where you open your mouth wide, take a deep breath in, and then let it out slowly. Your jaw stretches open, your eyes might squeeze shut or water a bit, and for a few seconds you can't really control it. Everyone yawns: babies, grandparents, dogs, cats, even fish and birds.

We usually yawn when we're tired or bored, though scientists still debate exactly why we do it. One theory suggests yawning helps wake up your brain by bringing in extra oxygen or cooling it down slightly. Another idea is that yawning evolved as a social signal, a way for groups of early humans to communicate that it was time to rest.

The strangest thing about yawning? It's contagious. If you see someone yawn, you'll probably yawn too. Even reading about yawning can trigger one (you might be yawning right now!). This happens because of how our brains mirror what we observe in others. Scientists think this contagious yawning might be connected to empathy, our ability to understand what others are feeling.

As a noun, a yawn can also mean something boring. If a movie makes the whole audience yawn, that's not a good sign for the filmmaker. People might even call it a yawnfest.