Picture this scene:

As you prepare to feed a parking meter, a coin drops from your hand and lands on the sidewalk. You reach down, pick it up and push it into the coin slot. An effortless motion.

Yet picking up a coin, buttoning a shirt and many other common tasks would be virtually impossible without fingernails.

Considered part of the skin, nails are resilient shields, tough enough to protect the underlying tender tissue against many irritating substances, including acid. These vestigial claws allow scratching and increase the sense of touch.

Without nails, "fingertips would be less sensitive," says Dr. Richard Scher, head of the section for diagnosis and treatment for nail disease at the Brown University Program in Medicine. This has been shown in people who have lost fingernails. "If you closed your eyes and tried to tell the difference between a nickel and dime, it would be a lot harder to do if you didn't have nails."

What most people call the nail is technically the nail plate -- a hardened protein layer of keratin that grows fast three millimeters a month on the fingertips. On the toes, the pace is a bit slower: about one to 1 1/2 millimeters a month. "What this translates to," says Scher, "is a new fingernail about every five to six months and a new toenail every 12 to 18 months."

Nail formation begins long before birth, at just 2 1/2 months of gestation, and is completed by fourth or fifth months of pregnancy.

The nail plates are formed from the nail matrix -- a collection of cells located beneath the nail near the cuticle. Most people can see at least part of the nail matrix on their fingers. Look for the half moon structure, or lunula, just above the cuticle.

Other parts of the nail include a nail bed that rests just below the nail plate and a nail fold about a quarter inch from the cuticle toward the first knuckle.

The nail plate contains no cells and no blood vessels, leaving some people to contend that it is not a living structure. But the nail plate "can expand, contract and get brittle," Scher says. "To me, it is alive."

Although it looks and feels far different from hair and skin, the nail plate is chemically very similar. Differences result from varying kinds of chemical bonds holding the amino acids -- the building blocks of protein -- together.

Like the rings of a tree, nails give clues to those who know how to read them. White marks "show a temporary injury to the nail matrix," Scher says, for instance, pushing back too hard on the cuticle during a manicure. "If you see a white spot about halfway up the nail, you can say that about three months ago, there was injury to the matrix."

Indentations in the plate indicate a similar injury, while totally white nails can warn of a number of illnesses, including liver disease. High fevers, mechanical injuries, such as catching a finger in a car door, and infections can also produce nail plate damage.

Nail polish probably offers some protection to the nails -- provided it is "not used excessively," Scher says. But nail polish removers are a different story. They dehydrate the nail plate, make it brittle and can cause it to crack. His recommendation: wear nail polish for five or six days. Remove it on the weekend, and don't reapply polish until Monday.

In general, diet seems to have little effect on nails. Gelatin, once touted as a way to grow stronger nails, actually "has no effect," he says.

Only extreme diet regimens seem to take a toll on nails. "On starvation diets, nails won't fall out, but they can become extremely brittle and thin," Scher says. "The shine and luster are lost."