Q. Why do people yawn? I know a young man who yawns a lot, even when he's not tired. What could be causing it?

A. Everyone is familiar with some of the situations associated with yawning -- tiredness, boredom or just seeing someone else yawn. Infants yawn when they are only a few days old. Dogs, cats and even crocodiles yawn, though the reason is something of a mystery. But here are a few ideas about what makes people yawn.

Scientists have thought that for some people yawning may be a response to lack of oxygen or glucose, the fuels that supply energy to the brain. The idea is that yawning temporarily improves the blood supply to the brain and re-energizes it. Some people, for instance, yawn when they get hungry.

Other theories are that yawning has something to do with the sense of smell, and that early man used to yawn to "sniff" the air around him for predators before going to sleep at night or after waking up in the morning. This theory is one of the few that offers any explanation about why people yawn in a chain reaction.

Yawning also seems to help our lungs work better, by opening up the tiniest air passages with a deep breath. This maneuver would be especially beneficial before bedtime, when our breathing becomes more shallow. Other causes of yawning are drugs that depress brain function, such as alcohol, tranquilizers or narcotics.

Some researchers have proposed psychological causes of yawning, saying that it is an expression of mood, like laughing or crying. In this regard, yawning may be a way of saying, "I'm bored." In some cases, yawning is a somewhat veiled sign of other emotions, such as anxiety (taking a final exam), fear (waiting for the dentist) or restrained aggression (being cornered by an obnoxious person at a party).

In some circumstances, yawning may simply be an indicator of mental alertness, a sign that someone is trying to focus his attention on the outside world. In classrooms, teachers often prefer students who, though yawning, are trying to follow their lecture to wide-eyed students whose thoughts are elsewhere.

Why yawning is contagious is puzzling, though it seems to happen more readily if you identify with the person who starts yawning. You're probably less likely to join someone in a yawn if you have no interest in that person.

Since science hasn't come up with any generally accepted notions about why people yawn, I'd be interested in hearing from readers any pet theories on this strange phenomenon. I'll print the answers in an upcoming column.

Follow-Up: Hemorrhoids

Q. In your recent answer about hemorrhoids, you discussed surgery for severe cases. I'm considering having surgery for my problem hemorrhoids. Could you describe types of procedures, cost, inconvenience, safety and effectiveness?

A. In my discussion, I mentioned that there are two types of hemorrhoids, internal and external. The key difference between them as far as surgery is concerned is that internal hemorrhoids can be operated on without general anesthesia, so surgery can usually be done in an ambulatory (in-and-out) surgery center or a physician's office.

Because of their more sensitive nerve supply, external hemorrhoids usually require general anesthesia and a hospital stay.

For internal hemorrhoids, you have several options: Banding, in which a small rubber band is placed snugly at the base of the hemorrhoid. Because this cuts off its blood supply, the hemorrhoid tissue shrivels up and falls off. Cryosurgery, in which the hemorrhoid tissue is frozen to the point of destruction. Sclerosing therapy, in which the hemorrhoid is injected with a chemical that turns it into scar tissue. Infrared probe coagulation, which turns the hemorrhoid into scar tissue by making the blood within it clot. Laser treatment, which also destroys hemorrhoid tissue but sometimes requires a longer time for recovery than other office treatments.

These procedures usually cause only minor discomfort and can be done in a physician's office. Generally, they won't interrupt your daily routine. They are safer, more convenient and less expensive than an operation requiring general anesthesia.

For external hemorrhoids, these procedures would usually be too painful. For severe cases, you can have surgery to remove the hemorrhoids. This operation usually will give you a sore bottom, keep you off your feet and keep you away from work for a week or two. On top of this are the increased costs of general anesthesia and a hospital stay.

All these types of surgery are generally effective, although there is a small chance that new hemorrhoids could form, possibly causing your symptoms to return.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.