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Sources:
http://www.upenn.edu/gazette/1109/expert.html
http://news.nationalgeographic.com/news/2011/11/111115-yawning-mystery-brains-sinuses-health-science/
http://serendip.brynmawr.edu/exchange/node/1912
http://www.loc.gov/rr/scitech/mysteries/yawn.html
http://www.smithsonianmag.com/science-nature/why-do-we-yawn-and-why-is-it-contagious-3749674/?no-ist
https://www.princeton.edu/main/news/archive/S31/64/49G23/index.xml?section=newsreleases
http://www.psychologytoday.com/blog/brain-sense/201111/why-do-you-yawn-when-youre-not-sleepy
http://www.epjournal.net/wp-content/uploads/ep0592101.pdf