Cut up an onion, chuck it on the barbie, bung it on a sausage, and you’re in for a real treat! The problem is, when you cut the onion up it hurts your eyes and makes you cry.
This happens because when you slice the onion open you also break some of the onion cells. This result in some substances in the onion mixing together, enzymes and sulfur containing compounds to be precise. When these mix a chemical reaction happens which releases a gas. This gas reaches your eyes, mixes with the water that keeps them moist and then another chemical reaction occurs. This reaction produces, among other things, a mild sulfuric acid, which irritates the eyes. Your eyes are very sensitive so they produce tears to wash away the nasty acid and protect your eyes. And it is no good eating a snag off the barbie with tears running down your face.
Fortunately a bunch of geneticists have come to the rescue and created tear free onions. They genetically modified the onion so that the gene (bit of DNA) which produces those enzymes is switched off. This means no more nasty gases and no more tears. Read more here or here.
Hurrah! Sausages with onion for everyone 
