Onions have a defense mechanism. No, they don’t have impenetrable steel armor, or spikes, or adamantium skeletons. They rely, instead, on chemical weaponry. Now, before you go calling the Department of Homeland Security on your green-grocer, hear me out. Inside the cell walls of a typical onion are a collection of enzymes called "allinase" and "prensco." Normally separated, once you start to cut an onion, those walls breakdown, and those enzymes create 1--propenylsulphenic acid, which further break down into propanethial S-oxide gas.