The old belief was that people who are short of oxygen would take in these great amounts by yawning. The latest theory, which makes more sense to me as I know that we breathe in well enough, is that there is a build up of excessive carbon dioxide in our lungs, (because we are not exhaling to the full potential), so we are forced into taking in a large amount of air, and this in turn forces out a good amount of carbon dioxide. Sometimes we have to do lots and lots....