HOW COME? What happens in a blink of an eye

How come if we stop ourselves from blinking, tears come out of our eyes? asks Jorge Oidor, a student in Queens.

Ever hear the phrase "Blink, and you might miss it?" No worries. Each blink of an eye lasts less than 4/100ths of a second. Most of what's going on around us is happening slowly enough that we won't, in fact, miss it. However, force your eyes to stay open, and the resulting irritation and tears may keep you from seeing clearly. (And you might actually miss something.)

Although we don't usually notice it, our eyelids close and open about every six seconds, for all the hours we're awake. The main cause of all the frantic winking: A dusty planet and dry air.

What happens during a blink? The eyeball retracts, pulling back a bit less than a millimeter. And as the lid closes, it acts like a windshield wiper, spreading soothing substances (tears, oil, and mucus) over our delicate eyeballs.

The tears well out from the lacrimal glands in the eye's inner corner; they bathe the eye in watery, slightly salty fluid. The oil oozes from sebaceous glands lining the edge of the eyelash-studded lid. Meanwhile, the mucus seeps from the eyelid's mucus glands. So with each quick blink, the eye's transparent cover, the cornea, is cleaned, moistened, and lubricated. Thwart the blinking, and the result may be a rush of cleansing tears.

Thankfully, we don't have to think about the every-few-seconds blinking, or we'd get nothing else done. Like breathing, or the pupils' widening and narrowing response to light, blinking is an automatic reflex, controlled by the body's central nervous system.

Besides protecting vulnerable eyes from a dry, dusty environment, blinking also acts as a physical shield against injury. Notice a bug (or a ball) flying toward your face? Your blink reflex kicks in, closing your eyes against the threatening impact.

Studies show we also blink more when we're talking or excited than when we're quiet or relaxed. Our blink rate may drop to a slow-mo three times a minute when we're concentrating on a task, from solving an equation to reading a book to surfing the Internet. (Which is one reason why staring at a computer can leave eyes feeling so irritated.)

Luckily, although we may blink 600 times an hour, we don't notice each split-second of sudden darkness. Each time a lid descends like a window shade, the brain's visual system hangs on to the last image we saw -- until the eye's shade flies up again. This "persistence of vision" allows us to watch the world around us (as well as TV and movies) without missing crucial pieces of the action.