How often and why do people's eyes blink?
A blink lasts about a 10th of a second, and most people blink about 15 times a minute, or every 4 seconds.
Obviously there is some cleaning and lubrication of the eye that gets handled by blinking, and you certainly know that if you get something in your eye you'll start blinking fiercely. You can also experiment with stopping yourself from blinking and will find after a while that it's a bit uncomfortable. A blink can also be a protective mechanism against bright lights, as any photography buff is only too well aware! Similarly, you may well blink in the presence of irritants like freshly cut onions or tear gas.
Blink rates vary quite a bit depending on emotional and mental states. Stress and anxiety tend to increase a person's blink rate, and this can sometimes give away a lie. Intense concentration tends to reduce the blink rate, and if you're in a situation that implies some danger, your blinking rate can go way down -- presumably to help you look around quickly without missing things.
A really interesting thing that's not well understood, but very easy to notice once you know to look for it, is that when you change from looking at something to thinking about it, you tend to blink -- a sort of signal that you're changing mental gears.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to firstname.lastname@example.org or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.