IT starts with a tingling, itch-like sensation at the back of the mouth, develops into a deep intake of breath, followed by an equally resonant exhalation -- often to the accompaniment of a satisfying groan.
To the medical profession it is a "brainstem-mediated bodily response"; to anyone else it's a yawn.
Most animals with any backbone yawn spontaneously but only humans, chimps and possibly some species of monkey suffer from contagious yawning -- when the sight or even thought of someone else yawning starts a chain reaction.
Why yawning should be infectious has foxed some of the greatest minds in science but the latest study into the topic suggests it may have something to do with emotional empathy -- we yawn when we see someone else yawning because of our need to empathise with other people.
Atsushi Senju of Birkbeck College in London and a team of Japanese colleagues investigated infectious yawning in autistic and non-autistic children.
They found that, unlike ordinary children, autistic children did not experience contagious yawning.
Autism is known to be a developmental disorder of the brain which results in children being unable to form the normal emotional ties with the people they meet.
They have an inability to empathise with others.
The findings suggests that autistic children do not respond to contagious yawning, and that emotional empathy may be the underlying reason why yawns are infectious.
"It supports the claim that contagious yawning is based on the capacity for empathy," Dr Senju said.