Sneezing involves a sudden and violent expiration of breath through the nose and mouth. It is a simulation of nerve endings, which means a message is sent to the brain causing a reflex action called sneeze. Sneezing usually occurs when the sensitive lining of the nose is irritated. It starts with chemicals like histamine being released by inflammatory cells in the nose, telling the body that there is something that shouldn't be.
Like many other important functions of the human body, sneezing is a way our body protects us from unwanted and bothersome things. It is a powerful way of ridding

the nose of unwanted germs and dirt. These unwanted intruders can include a bit of pollen, hair, perfume, cold air, a viral infection, smoke or anything that can fit and tickle the nasal passages.
However, in allergies our body gets it wrong and thinks that something harmless (such as 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 powerful by closing the vocal cords and building-up pressure inside the chest. Many parts of body like the throat, chest, vocal chords and more work together to make the sneeze as powerful as possible. The way that these parts cooperate to build up force and to open and close at just the right moment gives the sneeze that extra power it needs!
Air in the chest is under lots of pressure so when the vocal cords are opened, it rushes out really fast (like how air in a balloon rushes out once you let it go out of the mouth after blowing it up). This result in anything unwanted being shot out of the nose at great speed!

Etiquettes on Sneeze
Do not forget to hold your hand or a tissue in front of your nose and mouth when you sneeze! It is not very healthy to accidentally let the germs and other things that come out flying at super-sneeze-speed hit someone else who is standing near by!