Here’s one for the docs (or even the paradox):
Why do my eyes water when I whack my thumb with a hammer? It’s not necessarily the thumb, of course; I can think of many other body parts that make my eyes water when they get impacted. But you get the point.
I’ll note that eyes water for many reasons: emotion, of course, or to clean out the eye. But simply due to pain? What gives?
I don’t know for sure but here is one possibility. There are itty bitty drain ducts in the corner of your eyes that ordinarily take the tears back down into your throat somewhere.
Maybe when you smack your thumb a surge of adreneline raises your blood pressure enough to close off these ducts.
I know by experience that these ducts tend to close up when you get to be my age. That’s why old people often have watery eyes.