While yawning is often associated with being bored, scientists say that idea doesn't quite stack up.
The act of involuntarily opening our mouth wide and exhaling deeply is something humans do on a daily basis, and there are a range of theories as to what triggers it.
The Sun looked at four of the possible reasons:
The physiological theory
Scientists often reference the idea that we yawn because out bodies are trying to draw in oxygen to remove a build up of carbon dioxide.
This theory could explain why we tend to yawn when there is a group of people in a room, but it doesn't shine any light on why we also sometimes yawn when we exercise.
And a neuroscientist at the University of Maryland, Robert Provine, added more doubt to the theory. His research found that giving people more oxygen didn't stop yawning, and decreasing the amount of carbon dioxide also made little difference.
The theory of evolution
Some experts believe the humble yawn is a throwback to our ancient ancestors, who used to show their teeth to intimidate others.
Others say yawning was developed in caveman times as a subtle signal to switch activities.
The theory of boredom
This is the theory most common in pop culture, but experts say it's unlikely to have any truth to it.
After all, it doesn't explain why an Olympian might yawn before taking part in a big event, or why it is that dogs yawn before they're about to attack.
The brain-cooling theory
A more recent idea is that people yawn more frequently when their brains are likely to be warmer.
In a test, scientists asked people to breathe through their nose while a hot or cold pack was held to their forehead.
The research linked cooler brains to clearer thought patterns, so it's possible the body triggers yawning to help keep us more alert.