Almost everyone yawns when they feel sleepy. So do most animals.But we don't completely understand why people and animals for that matter, yawn.

It's widely assumed that yawning occurs because we are tired or bored or because we see someone else doing it, but there isn't any hard evidence to support these beliefs.

Scientists do notpurportto know all of the biological mechanisms of the yawn, but tend to agree that a yawn is aninvoluntary respiratory reflex, which regulates the carbon dioxide and oxygen levels in the blood.

Technically, a yawn is the reflex opening of the mouth followed by the deepinhalationand slowexhalationof oxygen.

The very act of yawning is but one of a number of involuntary reflexes controlled by thespinaland nerve centers.

Scientists speculate that the onset of a yawn is triggered either by fatigue, or by sheer boredom as, at those times, breathing is shallow, and little oxygen is carried to the lungs by the oxygen-totingcardiovascular system.

When one yawns, his or her alertness is heightened, as the sudden intake of oxygen increases the heart rate, rids the lungs and the bloodstream of the carbon dioxide buildup, and forces oxygen through blood vessels in the brain, while restoring normal breathing and ventilating the lungs.

This quite plausible theory of yawningfalls short ofexplaining many aspects of yawning. Scientists explain away the "contagious" nature of yawning, that is when one person's yawn triggers another nearby to yawn, as due to the power of suggestion, but areat a losswhen attempting to explain why yawning occurs excessively in patients with lowerbrainstemdamage or with multiplesclerosis.

Other unlocked mysteries include whyfetusesin thewombyawn, when it is a well-known fact that they do not intake oxygen into their lungs until after live birth, or why individuals with high concentrations of oxygen in their blood streams yawn.

Until these questions are answered, do not assume that a person who yawns in your presence is bored with what you are saying, or suffers from exhaustion. Simply be pleased that he or she is not bored to death.