Be quiet for a minute. Sit very still and listen. What do you hear? Are there cars going by outside? Is someone playing music in another room? Is there a siren wailing in the distance? Do you hear laughter or birds singing?

You hear these things because your ears are sensitive to sound. They pick up vibrations called sound waves that travel through the air. Then your ears translate the sound waves into signals that your brain understands. Your brain identifies the signals as a voice speaking, a baby crying, a radio playing your favorite song or whatever other sound you heard.

But not everyone can hear sounds. Some people have problems that interfere with this sense. People with hearing problems may hear sound faintly, or the sound they hear may be distorted. And some people cannot hear any sounds.

According to the National Information Center on Deafness, 20 million Americans are hearing-impaired. That's one in every 11 people. Of those 20 million, 2 million are considered deaf, which means they cannot hear at all.

When the sense of hearing works, here's what happens:

The outer ear -- the part you can see on either side of your head -- collects sound. The sound passes into the ear canal and starts vibrations in a little piece of stretched skin called the eardrum. Three tiny bones in the middle ear pick up vibrations from the eardrum and conduct them to the inner ear.

Now things get pretty complicated. In the inner ear, there's a spiral-shaped chamber called the cochlea. This snail-shaped part of the inner ear changes the vibrations that start in the eardrum into electrical signals that stimulate hearing nerves. The nerves send signals to the brain.

The cochlea is carved right into the skull. It's filled with liquid that is very sensitive to vibration. Vibration that starts out in the eardrum sets off waves in the fluid. Then the moving fluid stimulates special cells that line the cochlea. There are more than 16,000 of these cells, shaped like tiny hairs. As they wave back and forth in the moving fluid, they generate electrical current in the hearing nerves. The nerves transmit the signals to the part of the brain that's in charge of hearing.

In a system this complex, several things can go wrong, causing hearing loss or deafness.

Deafness may be caused by injuries or illnesses that change the structure of the ear. Sudden, extremely loud noises or repeated loud noise over a period of time can hurt the ears, too. Or the hearing problem may be something a person is born with. Worldwide, about one person in a thousand is born deaf, and doctors don't always know why.

Some people develop diseases or physical problems in the parts of the ear that carry sound: the eardrum or the tiny bones in the middle ear. Sometimes, these problems can be corrected with surgery; other times, hearing aids can make sound louder.

A different kind of hearing loss happens when the delicate inner ear is damaged. If the tiny hair cells lining the cochlea don't work, they don't stimulate the hearing nerves. So the nerves no longer transmit messages to the brain.

But modern technology can help some people with this kind of hearing loss. Over the last few years, doctors have developed an instrument called a cochlear implant. This electronic device can bring back partial hearing to some people who are totally deaf. Unlike regular hearing aids, which simply make sound louder, a cochlear implant directly stimulates the nerve cells that communicate the message "sound" to the brain. So far, about 3,000 people in the U.S., including some children, have received implants.

A cochlear implant has several parts. There's a microphone to receive sound, a computer to translate it into electrical signals, and electrodes to send those signals to the brain. Part of the device is worn behind the ear like a regular hearing aid. Part is implanted in the bone by the middle ear. The internal part directly stimulates the hearing nerves to send sound signals to the brain.

Cochlear implants do give people who use them information about what's happening in the world around them, but they do not provide normal hearing. Most deaf people who receive implants are able to detect medium to loud sounds and to hear the rhythms of speech but not words or sentences. However, implants make it easier to lip read, and some people are able to understand words and sentences. Tips for Parents

The cochlear implant is designed only for people who receive little benefit from conventional hearing aids. Candidates for use of the device are age 2 or older, according to the American Academy of Otolaryngology-Head and Neck Surgery. Results vary widely, the academy cautions. To receive a free leaflet entitled "Cochlear Implant: A Device to Help the Deaf Hear," send a self-addressed, stamped business-size envelope to: The American Academy of Otolaryngology-Head and Neck Surgery, One Prince St., Alexandria, Va. 22314. (703-836-4444.)

Catherine O'Neill is a children's writer.