Sneezing
is set off by many different things.

Tina
Franklin / Flickr

Aside from allergies, you can sneeze at seemingly random
things. I sneeze every time I start chewing a piece of gum.

It happens to other people whenever they look at a bright light or toward the sun, or
even when they pluck their eyebrows or have sex. 

So why does this happen?

It turns out that sneezes start in your nerves. They are one of
your body’s ways of keeping out irritants in your nose and
throat, according to a
post on the blog Penn Medicine News, which is
maintained by the University of Pennsylvania.

“It’s a nerve transmission that tells your brain something is in
your nose that needs to come out,” Dr Neil Kao, an
allergy and asthma specialist at the Allergic Disease and Asthma
Center in Greenville, South Carolina told WebMD.

When something enters your nose, it sets off the “sneeze
sensor” in your brain, which then sends signals for you to
to close your throat, eyes. and mouth. Next, your chest muscles
contract and your throat muscles relax. This forces air
(along with anything else) out of your mouth and nose. That’s a
sneeze.

However, our noses don’t always get it right, and sometimes our
body mistakes harmless things as an attack. 

Strange sneeze triggers:

• Plucking your eyebrows is a common culprit.
  Dr. Melanie
  Grossman, an assistant clinical professor of
  dermatology at Columbia University,
  told New York Magazine’s The Cut that tweezing may set
  off the trigeminal
  nerve, which passes sensations between
  the brain and the face.
• Exercise can set some people off on a sneeze
  spree. You hyperventilate when you’re
  over-exerted, which means your nose and mouth start to
  dry up. Your nose compensates for this by drippings, which
  triggers a sneeze.
• Bright sunlight causes sneezing for a third of people.  This light
  sensitivity is an inherited trait and is known as the “photic
  sneeze reflex.” There’s no hard evidence for why it happens,
  but some research suggests that the
  reflex that makes your pupils dilate and the reflex that makes
  you sneeze are connected in light-sensitive people. 
• Sex may make some people sneeze too.
  A 2008 review in the Journal of the Royal Society of
  Medicine called
  it an “underreported phenomenon.” Researchers
  think that the stimulation of the parasympathetic nervous
  system — part of the nervous system responsible for
  the stimulation of “rest-and-digest” or “feed and breed”
  activities — fires off signals that makes people sneeze either
  when they’re thinking about or orgasming from sex.
• Alcohol. Some people sneeze after
  drinking alcohol, as well as other nasal symptoms. Doctors
  think this is probably because blood vessels in the nose
  dilate, resulting in mucus production, known as a form of
  non-allergic rhinitis.
• Chewing gum. I can’t find a definitive answer
  for this one, but it seems to affect a fair few people. One
  answer I’ve got is that the vapour given off by minty flavours
  is very powerful, and this tickles the inside of your nose,
  triggering a sneeze.