derives from the Old English words 'Ginian' and 'Gionian' meaning to
"open the mouth wide, as if to gape." It is a reflex action of
simultaneous inhalation of air and stretching of the eardrums, followed
by exhalation of breath. In man, it's commonly associated with
tiredness, stress, overwork, lack of stimulation and boredom, though
recent studies show it may be linked to the cooling of the brain.
However in animals, there are a number of reasons why certain animals
take to yawning.