Yum!! breakfast of champions!!! Okay, okay really, I'm so wanting to know what onions have to do with chemistry!
Glad you asked!
So why do onions make you cry when you cut them?
Chopping them releases tiny aerosol droplets that you can smell and that also end up in your eyes! The droplets contain a powerful sulfur containing irritant: syn-propanethial-S-oxide.
Propanthial S-oxide is known as LF or lachrymal factor because it stimulates your lachrymal ducts. For crying out loud I mean it makes you cry
(http://www.food-info.net/uk/qa/qa-fp133.htm)
Seems pretty obvious to a chemist. Best thing to do? Keep the safety goggles handy... ; )
Or run the onion under water and keep the surface wet to cut down on the aerosols. Of course using onions grown in sulfur poor soils will be milder.