Tears of the people are all the time to produce, however, it is very little, so you will not feel the tears flowing. In fact, not only yawning, laughing, sneezing, coughing, vomiting, shouting and so on, also can led to tears. Most are also the same reason, these actions are accompanied by a very large mouth open and a lot of air exhaling from the body to exhale carbon dioxide and inhaling oxygen. After a series of reactions, the tears cannot go on, more and more are produced in the eyes. Go on like this, you will be tearful, and finally you suffer the tears and cannot stop.