A big yawn involves much more than opening your mouth.
Muscles around the eyes, cheeks, jaw, and face all move. Your eyelids often squeeze partly or completely shut.
That movement can temporarily change the way tears are spread across the eye and how they drain away.
The result can be a little spillover.
Your eyes are always making tears
Tears are not produced only when you cry.
Your eyes maintain a thin tear film all day. It helps keep the surface moist, smooth, and protected.
Blinking spreads that fluid across the eye and moves excess tears toward tiny drainage openings near the inner corners of the eyelids.
From there, much of the fluid eventually drains toward the nose.
What changes during a yawn?
Yawning stretches and contracts several facial muscles.
The eyelids may close tightly, and the tissues around the tear drainage system can shift or become compressed.
For a moment, tears may not move through their usual route as efficiently.
If enough fluid collects, some of it can spill over the edge of the eyelid.
Does yawning squeeze the tear glands?
Facial movement may put pressure around the tissues involved in tear production and drainage, but the important point is not that yawning simply “turns on” sadness tears.
The mechanical changes around the eyes can alter where existing tear fluid goes.
Different people tear up by different amounts, and some barely notice the effect.
Why do your eyes and nose seem connected?
Because they are.
The tear drainage system empties into the nasal area. That is one reason crying can also make your nose run.
When you yawn, the same general plumbing is involved, although usually on a much smaller scale.
Is watery-eyed yawning a sign something is wrong?
Usually not.
A small amount of tearing during a yawn is common.
Eyes that water constantly, become painful, stay very red, produce unusual discharge, or show major changes unrelated to yawning can have other causes.
But a few tears during a giant end-of-day yawn are generally just part of the facial mechanics.
So your yawn did not suddenly make you emotional.
It moved the muscles around your tear system.
Apparently being tired was not inconvenient enough by itself.
The same body systems connect with Why do we yawn?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why do we yawn?, Why does your nose run when you cry?, and Why do we have eyebrows?