Because human tears are secreted all the time and flow on the surface of the eyeball. However, there are very few tears secreted at ordinary times, and they flow closely along the surface of the eyeball and the tiny gaps inside the eyelids, so you will not feel the tears flowing.

However, the moment you blink, the tears on the surface of the eyeballs are sucked into the sewer of tears-the nasolacrimal duct. Don't underestimate this thin tube, which is not too long, but it is the only way for tears to reach the nasal cavity. The lacrimal glands continuously secrete tears, and the nasolacrimal ducts continuously discharge tears, so people usually do not shed tears.

When a person yawns, he or she opens the mouth very wide to allow more air to escape from the body, in order to exhale carbon dioxide and inhale oxygen. A puff of air was forcefully exhaled from the mouth as he yawned. The muscles in the cheeks, tongue, and throat contract tensely, and the pressure in the mouth and nasal cavity increases. The increased pressure in the nasal cavity will temporarily block the nasolacrimal duct's work of excreting tears. The "sewage" is blocked, tears cannot flow, and more and more tears accumulate in the eyes. Tears would well up in his eyes, and finally they would burst out of his eyes and flow down his face.

In fact, not only yawning will cause tears, but also when people laugh, sneeze, cough, and vomit. The reason is the same.

When people are sleepy, they often yawn continuously to remind the body that the brain is tired and needs sleep, so yawning is also a method of hypnosis.