Why do Onions Make You Cry?

Cutting onions is arguably the worst part of cooking, the burning irritation you get in your eye becomes unbearable so much as to take a break mid way through chopping.  So why is this? This is all to do with mixing molecules in the onion cells that aren't supposed to be together, an enzyme named Allilinase and a molecule called Allilin.  Think of this like the type of adhesive that has two parts that you need combine together to make the glue active.  When we cut onion we start the chain of reactions allowing the Allilin to come in contact with Allilinase to turn the Allilin into Sulfenic Acid and this Sulfenic Acid turns into syn-Propanethial-S-oxide though the enzyme Lachrymatory Factor Synthase.

This syn-Propanethial-S-oxide is what everyone hates because this is the irritant that when combined with the moisture in your eyes gives you that pain and your body tells your tear ducts to try and wash it out.

References:
Something about Science

How Bioluminescence Works

Bioluminescence is the ability of a living organism to emit light; a phenomenon which has intrigued humans for thousands of years and allowed them to invent mystical and magical stories behind these beacons of light. We might have decided that these lights serve a magical purpose, but evolution has found many uses for this process from protection, all the way to communication.  The chemical mechanism behind these bioluminesent photons is a very simple one, though not the same in all organisms, they share some of the same basic principles.  Taking the firefly as an example, the compound which acts as it's light bulb is called luciferin, and the reaction begins with molecular oxygen and ATP, with the help of the enzyme luciferase, turning luciferin into a dioxetane derivative (mechanism of this can be seen here)¹.

Once this dioxetane derivative is formed the emission of light reaction is ready to occur spontaneously. This happens in a concerted reaction releasing CO₂ and producing an excited ketone. The electron of the excited ketone then falls in energy to create a photon and end with the final product of oxyluciferin.

The newly created oxyluciferin is then enzymatically recycled into luficerin by this mechanism here².

References:
1.  Aldo Roda Chemiluminescence and Bioluminescence: Past, Present and Future, p. 57, Royal Society of Chemistry, 2010, ISBN 1-84755-812-7
2.   Keiko Gomi and Naoki Kajiyama. Oxyluciferin, a Luminescence Product of Firefly Luciferase, Is Enzymatically Regenerated into Luciferin. J. Biol. Chem. 2001 276: 36508-36513. July 16, 2001, doi:10.1074/jbc.M105528200