Yaaaaaaawn. When it comes to yawns, two things are certain: Everybody yawns — and nobody really seems to know why.

We begin yawning very early in life — before we’ve even entered the world, to be exact. Ultrasounds have discovered that fetuses begin yawning and hiccuping as early as 11 weeks into their development.

There are several theories about why we yawn. One theory suggests that we don’t breathe as deeply when we’re bored or tired as we do when we’re excited or exerting energy.

When our breathing slows, our bodies receive less oxygen. This reduction in oxygen makes us feel tired and lethargic.

Yawning helps bring a burst of fresh oxygen into the blood while pushing carbon dioxide out. If this theory is correct, yawning is an involuntary reflex.

More recent research suggests yawning may be a way our bodies distribute oily substances called “surfactants” that lubricate the lungs and prevent them from collapsing as we exhale and inhale.

No matter which yawn theory you believe, one thing is for sure — yawning is contagious! If you’ve ever watched someone else yawn, it’s likely you’ve caught the yawn, too!

Scientists continue to study the “contagious yawn” phenomenon, but they’ve yet to conclude exactly what causes humans to yawn simply because they see another person yawning. Interestingly, their research has found that, like humans, chimpanzees will yawn when they see another chimpanzee do so.

The next time you get an urge to start a trend, just open up your mouth and yawn. You may be surprised to find the person next to you can’t resist the urge to join in, too.