It is popularly believed that yawning is caused by boredom or tiredness. New research suggests that there might be other reasons, but so far only theories. One of then is that a yawn helps us gulp in a huge lungful or air and get a large amount of oxygen and release a large amour of carbon dioxide. Another theory says we yawn to stretch out our muscles, especially the tongue and throat muscles. Actually most times we yawn and stretch out :). Research will soon tell us the truth.