Onions may be delicious to cook with but cutting them can cause your eyes to water. How do you avoid shedding tears? First, let's look at why onions make our eyes water. When the skin of an onion is broken, the onion has a defense mechanism, which emits enzymes and sulfenic acid that produce the gas propanethial S-oxide. Propanethial S-oxide turns into sulfuric acid when it touches the water layer that covers and protects the eyes. When your eyes detect the gas, they defensively react by generating tears to flush it out. Here are 5 hacks to fight back the tears.