Your eyes blink, or close the eyelids rapidly, for any of several reasons. It might be to protect themselves from an irritating substance or to protect themselves against a bright light or to keep themselves clean.
Every time you blink your eyes, you are really crying, or producing tears. Under your upper eyelids are tear glands, the lachrymal glands. Each time your eyelids close, the movement creates a suction, which draws out some salty fluid, or tears, from the lachrymal glands. These tears keep your eyes from drying out.
But if an irritating substance, such as a speck of dirt, reaches the eye, the lids automatically blink and tears appear to wash out the eye and protect it against the irritant. These tears drain from your eyes into your nose through your tear ducts, tiny tubes at the inner corner of each eyelid. When the tears are too great to drain through the tear ducts, the ducts become flooded and the tears run down your face.
You blink your eyes about 25 times a minute!