Whether it's caused by a cold or flu, hay fever or even a big whiff of pepper, the mechanism of a sneeze is the same.

Sneezes are "a reflex," says Dr. Eugene Robin, professor of medicine and physiology at Stanford University's School of Medicine. They begin with irritation to nerve endings in the nose. These nerves fire off a message to the brain, which then "signals the respiratory muscles in the chest to take a deep breath," Robin says.

What follows is an "almost uncontrollable expiration of the air," that moves through the nose and the mouth at velocities that may approach the speed of sound.

Sneezes protect the nasal passages, says Dr. Robert Naclerio of Johns Hopkins Medical Institutions, by "clearing out whatever is irritating it." They can be stimulated by dust and pollen or by larger objects, such as a pencil.

Why cold viruses cause sneezing is not known, Naclerio says. One theory is that the cold virus prompts the production of some kind of chemical mediator in the nose that triggers the nerves.

Even though sneezes are a reflex, they can be aborted. The book "The Nose," edited by Drs. Donald F. Proctor and I.B. Andersen (Elsevier Biomedical), reports that an unwanted sneeze can be stopped by applying pressure to the upper lip or nose.