It is believed the watering of the eyes when you yawn may result from pressure on the main tear glands.
These gland are at the outer margins of the eye sockets, and may produce tears because of the facial contortions involved in yawning.
The involuntary act of yawning usually includes opening the mouth very wide while slowly taking in a deep breath.
The same contortions might also put pressure on the salivary glands, especially in a stifled yawn, when the yawner struggles to keep the mouth closed while opening the throat wide.
There are three pairs of salivary glands: two over the angle of the jaws.
Two in the floor of the front of the mouth, and two toward the back of the mouth close to the sides of the jaws.
It is still not known exactly why people yawn or why yawning is contagious.
One theory suggests that yawning is triggered by increased levels of carbon dioxide in the blood, but other studies have contradicted this.