Because we get irritation in our noses; it’s the main organ that helps us breathe air. But, along with air comes a load of other stuff as well; dust, aftershave and perfume, pollen, cigarette smoke, and lots more besides.
There are thousands upon thousands of hairs in our noses called cilia; they help keep a lot of these things out of our noses (they’re not quite 100% successful, but almost). When the cilia get irritated, they send a signal to the sneeze centre in our brains.
The sneeze centre then coordinates all the different muscles in the body responsible for a sneeze in order to push whatever it is out of our nose. More parts of our body are involved in sneezing than you might realise; some of the muscles include stomach muscles, the chest muscles, the diaphragm, and the vocal chords (muscles control them too). All of these groups working together make us sneeze.
Oh yes, and we mustn’t forget the eyelids. Did you know that we can’t sneeze if our eyes are open? We simply can’t do it.
Anyway, all these muscles work together, and they push out the irritation from your nose at high speed; some particles can come out at up to 100 miles per hour!
Some people sneeze when they are in direct sunlight; if this happens, they are a photic sneezer. Their sneezes happen because they’re reacting to the sudden bright sunlight when stepping out from the shade.
Finally, one thing you should never do is try and stop (“stifle”) a sneeze. It can damage blood vessels and your ear drums – not very nice – so just let the sneeze flow!
PS – Did you know that you can’t sneeze when you’re asleep!