Sneezing is our body's way to itch our nose. A sneeze, which is also referred to as sternutation, is generally caused by an irritation or itch inside the nose.
When the interior of one's nose gets a slight irritation or tickle, signals are sent to the brain, and several muscles, including abdominal muscles, the diaphragm (a muscle under one's lungs that helps them breathe), and muscles in the back of the throat.
The eyelid muscles should also receive an honorable mention, since almost every time someone sneezes, his or her eyes snap shut. However, this is not always the case due to the fact that blinking during a sneeze is simply a reflex. The nose and eye muscles are connected by cranial nerves, and the nerve impulses coming from the nose oftentimes proceed on towards the eyes, causing them to close.
All the muscles cooperating in the process allow for a sneeze to propel its way through the nostrils at speeds sometimes reaching up to 200 miles per hour. No surprise the itches go away pretty fast. Common things that can cause sneezing include peppers, cold air (hence all the sneezing in the winter), or dust. When a common cold is caught, it causes swelling on the inside of the nose, that persists for some time. Other times, a sneeze is triggered from an allergic reaction, many coming from flowers, animals, foods or commercial products.
Some people also have a sneeze caused by bright lights, oftentimes the sun. This happens to about every 1 in 3 people, and is generally a genetic trait. This symptom is referred to as "photic sneezing," and is actually considered dangerous for military pilots, and is probably just about as lethal as the rest of what being a pilot entails. Not.
Another common sneezing behavior involves holding the sneeze in. Rumors state keeping a sneeze inside is "bad for you". However, this not necessarily the case; no deleterious damage is done physically. Yet, since the purpose of a sneeze is to eject any dust, pollen, etc. from the nostrils, keeping a sneeze in could prevent the body from getting rid of such particles, and could potentially lead to infection. The good news for all you stiflers out there: the infection is incredibly rare.
That's a lot for you to think about the next time you find yourself projecting pollen out of your nose at the speed of a racecar.