It's often believed that we yawn because we are bored or tired, but for a long time it was also thought the cause may be a lack of oxygen reaching the bloodstream. The yawn would then be a natural response of the body to this situation; if you think about what yawning is, it's really a very deep breath, a gulp for air. So when you yawn your body is really trying to increase the oxygen supply quickly.

However, later studies suggest that yawning isn't a very effective way of doing this. It's now thought that, as you tend to want to stretch when you yawn, that both together may be a way of keeping the body alert and ready for movement. This would also explain why you yawn more when you are tired, as the body would then need something to keep it on the alert.

One thing we do know - it's true that yawning is catching! Even thinking about it can trigger a yawn (did you yawn reading this?!)