Of all the involuntary actions carried out by the human body, nothing rivals the sneeze. A sneeze occurs in three stages. First, your head moves up and back as you take a sudden breath. (This is the “ah” part of the sneeze.) Second, you pause for a moment after your lungs fill with air. Third, your head moves forward and down as you contract the muscles in your chest, throat and abdomen. (This is the “choo” part of the sneeze.) In the process, you expel air and tiny droplets out of your nose and mouth at about 100 miles per hour. This is faster than a cheetah chasing down a gazelle on the African savanna.

The human body has thousands of nerves that help it interact with the world around it. Pain nerves in your feet let you know if you step on something that could hurt you. Nerves that are sensitive to temperature remind you to put on a jacket if it’s cold outside. Stretching nerves let you know if your bladder is full so you can find a bathroom — or a tree, depending on where you are.

The nose and sinus cavities are lined with small hairs, mucus-producing glands and hairlike microscopic structures called cilia. Hairs and nasal mucus trap dust, mold and germs. Cilia beat slowly to move foreign matter to your throat, where it can be swallowed. (The acid in your stomach destroys most of the viruses and bacteria that get swallowed throughout the day.)

Sneezing is a complicated reflex that’s designed to remove irritants from your nose and sinuses. When something “tickles” or irritates the nerves in your nasal passages, a signal is sent to the sneeze center in your brain. Your brain responds to this signal by orchestrating all of the muscular actions that are required for a sneeze to be effective. (Eye muscles aren’t needed, but you may have noticed that it’s impossible to sneeze without closing your eyes.)

The most common things that cause sneezing are colds and allergies. Other triggers include smoke, strong smells and animal dander (dead skin cells). Some people sneeze when they are exposed to light. This is called photic sneezing.

You should always cover your nose and mouth when you sneeze. If you forget, your mom or dad will remind you to do so. It’s important to cover up because an unchecked sneeze will spread germs all over the place. It would also be pretty embarrassing if you ended up with a big glob of snot on your upper lip.

The worst sneeze is the one that can’t decide if it needs to come out or not. I don’t know why this happens, but when it does, you get stuck between the “ah” and the “choo.” Sometimes a person can be stuck in “sneeze limbo” for minutes before the feeling goes away or you finally let one rip.

If you try holding back a sneeze by pinching your nostrils, your brain won’t explode. However, you may end up forcing air through your eustachian tube into the space behind your eardrum. This can really hurt, so unless you’re worried about a zombie finding you, never try to stop a sneeze!

—Howard J. Bennett

Bennett is a Washington pediatrician. His Web site, www.howardjbennett.com, includes past KidsPost articles and other cool stuff.