Why do we yawn? Nobody knows for sure.

Hippocrates had his own theory (it helped to release noxious air, particularly during a fever) and since then every century ventured its guess. Sure, it has to do with air and oxygen intake, but why is it then so contagious? Around 50% of people who observe a yawn will yawn in response. Its mere mention will trigger one.

We tend to yawn when we are tired, but also when we are under stress. Athletes before a race do yawn. For some researchers it helps to pump cerebrospinal fluid around the brain, which could trigger a shift in neural activity. Some other suggest that it may have the signalling role of helping to synchronize the behaviour of a social group.

Yawning might help to chill the brain and stop it overheating. The violent movement of the jaws moves blood flow around the skull, helping to carry away excess heat, while the deep inhalation brings cool air into the sinus cavities and around the carotid artery leading back into the brain.

Even unborn babies in the womb yawn. And surely you too did, reading these lines - didn't you?