Sneezing involves a sudden and violent expiration of breath through the nose and mouth. But what is the purpose of this, other than being a way for foul people on the train to cover me with snot when they don't bother covering their mouths?
Basically, sneezing occurs when the sensitive lining of the noise is irritated. And I don't mean when it gets a bit annoyed, but when something gets up there that shouldn't be there. It could be a viral infection, a bit of pollen, animal hair, perfume, cold air, smoke or pretty much anything else that will fit and can tickle your nasal passages.
A sneeze usually starts with chemicals such as histamine being released by inflammatory cells in the nose, telling the body that something is there that shouldn't be. Stimulation of nerve endings mean a message can get sent to the brain causing a reflex reaction that results in a sneeze. Of course, in allergies the body gets it wrong and thinks that something harmless (like pollen) is trying to attack us, so it causes the release of histamine and we start sneezing when there is no need.
A sneeze manages to be so powerful by closing the vocal cords and building-up pressure inside the chest. This causes that weird feeling just before a sneeze when you know you are just about to and generally end up with a really silly expression on your face. The air in your chest is under lots of pressure so when the vocal cords are opened, it rushes out really fast (think of how the air in a balloon rushes out if you let go of the mouth after blowing it up). This results in anything unwanted hopefully being shot our of our noses at great speed! Many people believe that if our eyes were to stay open during this process, they would be blown clean out of our head. If you'd like to find out about this you can read Can you sneeze with your eyes open?
Interestingly, sometimes people sneeze when there is no reason at all, such as when they look at bright lights. If you would like to find out more about this, please read What is the cause of sneezing fits?
