Glad to have more questions to answer- keep them coming!
And its a good one too!
Your nose is very sensitive- there are thousands of little detectors that allow us to smell. Pepper has a chemical compound called piperine in it- it makes up about 5-9% of it and is what gives pepper its smell. When this chemical gets on to the little detectors in the nose it panics them a little! The brain things that these are dangerous chemicals and wants to get rid of them and ACHOO!
So in a nutshell- sneezing is one way of getting rid of things your body thinks are dangerous to you and pepper seems a little dangerous to your nose.
And becasue I love crystals you can actually grow crystals of piperine starting off with normal pepper from a shop.
Another odd sneezing fact is that about 1 in 3 people sneeze when exposed to bright light- thats not dangerous, its just the brain getting a little confused!