A yawn is a reflex consisting of the simultaneous inhalation of air and the stretching of the eardrums, followed by an exhalation of breath.Yawning (oscitation) most often occurs in adults immediately before and after sleep, during tedious activities and as a result of its contagious quality. It is commonly associated with tiredness, stress, sleepiness, or even boredom and hunger. In humans, yawning is often triggered by others yawning (e.g. seeing a person yawning, talking to someone on the phone who is yawning) and is a typical example of positive feedback. This "contagious" yawning has also been observed in chimpanzees, dogs, cats, birds, and reptiles, and can occur across species. Approximately 20 psychological reasons for yawning have been proposed by scholars, but there is little agreement on the primacy of any one. During a yawn, the tensor tympani muscle in the middle ear contracts, creating a rumbling noise from within the head. Yawning is sometimes accompanied, in humans and animals, by an instinctive act of stretching several parts of the body, including arms, neck, shoulders and back.In animals, yawning can serve as a warning signal. For example, Charles Darwin, in his book The Expression of the Emotions in Man and Animals, mentioned that baboons yawn to threaten their enemies, possibly by displaying large canine teeth. Similarly, Siamese fighting fish yawn only when they see a conspecific (same species) or their own mirror-image, and their yawn often accompanies aggressive attack. Guinea pigs also yawn in a display of dominance or anger, displaying their impressive incisor teeth. This is often accompanied by teeth chattering, purring and scent marking. Adelie penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," opening their beaks and pointing their faces skyward. This trait has also been seen among emperor penguins. Researchers have been attempting to discover why these two different species share this trait, despite not sharing a habitat. Snakes yawn, both to realign their jaws after a meal and for respiratory reasons, as their trachea can be seen to expand when they do this. Dogs, and occasionally cats, often yawn after seeing people yawn and when they feel uncertain. Dogs demonstrate contagious yawning when exposed to human yawning. Dogs are very adept at reading human communication actions, so it is unclear if this phenomenon is rooted in evolutionary history or a result of domestication. Fish can also yawn, and they increase this behavior due to lack of oxygen.