Final answer:
Cutting an onion releases a gas called syn-propanethial-S-oxide, which turns into sulfuric acid when it comes into contact with the water in our eyes, causing a burning sensation and tears as a natural defense.
Step-by-step explanation:
Onions make us cry when we cut them due to a chemical reaction that occurs when the cells of the onion are broken. Inside an onion, there are compounds known as amino acid sulfoxides. When the onion is cut, these compounds are released and react with the rest of the onion to produce a gas called syn-propanethial-S-oxide. When this gas reaches our eyes, it reacts with the water in our eyes to form sulfuric acid. The sulfuric acid causes a burning sensation, and our eyes produce tears to dilute and flush out the irritant.
This is a defense mechanism for the onion, which evolved to deter animals from eating it. The use of a sharp knife can minimize the damage to onion cells and reduce the gas produced during cutting, hence, causing less irritation to the eyes.