Yawning has always been associated with feeling sleepy or bored or both, but in fact the latest thinking is that the yawning mechanism is to do with thermoregulation - in other words, we do it to cool our brains down.
Here's the science bit (we challenge you not to yawn as you read this).
When we're thinking, our brains use around 40 per cent of the body's metabolic energy and this can cause them to overheat.
Yawning involves taking a big gulp of air in, which travels through our upper nasal and oral cavities, cooling the mucus membranes and in turn the blood vessels in the brain.
Which is why, apparently, we yawn more in warm weather than we do in the cold.
So next time someone yawns while you're talking, try not to be offended. It could be that they've been thinking about what you're saying so much that their brain needs a breather.
Five more things you never knew about yawning
1. Even foetuses do it in the womb - sometimes as early as 11 weeks after conception.
2. Excessive yawning can indicate a brain tumour or heart attack.
3. Yawning can be a sign of empathy. The more you yawn when you see someone else yawning, the more empathetic you are, research has found. Those affected by autism or schizophrenia are less likely to do it, as they are less able to read social cues.
4. The older you get the less susceptible you are to yawning. But no one knows why.
5. All vertebrates yawn, including fish and reptiles and some animals use it as a means of communication.