When someone yawns publicly, people may think he or she is impolite—or at least extremely bored.
Rules of etiquette notwithstanding, yawning actually serves quite a useful purpose.
A yawn is an involuntary inhalation.
We usually yawn in the evening when we are fatigued by the day’s activities or in the morning after we wake up.
A deep yawn serves to increase our supply of oxygen and may refresh us momentarily; it is often part of the awakening process.
Probably the most impressive yawn of all is that of the hippopotamus.
This bulky creature can stretch open its huge cavern of a mouth an incredible 150 degrees!
The yawn enables an old bull hippo to show everyone else in the hippo pool who is boss.
It also serves as a toothy warning to any intruders who would dare trespass into his river domain.
