People's tears have been secreted and flow on the surface of the eyeball. However, there are few tears secreted at ordinary times, which flow closely along the surface of the eyeball and in the tiny gap inside the eyelid, so you won't feel the tears flowing.

However, in the blink of an eye, the tears on the surface of these eyeballs are sucked into the sewer of tears-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. Tears are constantly secreted by lacrimal glands and excreted by nasolacrimal ducts, so people usually don't cry.

When yawning, people's mouth is wide open, and the ruler may let more air out of the body so as to exhale carbon dioxide and inhale oxygen. With the action of yawning, a stream of gas exhaled forcefully from the mouth. Muscles in cheeks, tongue and throat contract, and pressure in mouth and nasal cavity increases. As a result of the increased nasal pressure, the work of discharging tears from the nasolacrimal duct will be temporarily blocked, the "sewer" will be blocked, tears will not flow down, and more and more tears will accumulate in the eyes. My eyes will shed tears, and finally tears will flow to my face.

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