Look a little closer
The tears that appear while chopping an onion have nothing to do with sadness. A knife breaks open onion cells, allowing substances and enzymes that were previously separated to mix. Their reactions produce an irritating compound that can travel through the air.
Onions use sulfur from the soil to make and store several sulfur-containing compounds. In an intact cell, these compounds are kept apart from an enzyme called alliinase. The moment a blade ruptures the cell walls, the ingredients meet and the chemistry begins.
Another enzyme then helps turn a reaction product into a volatile compound called syn-propanethial-S-oxide. Its name is complicated, but its job in this story is simple: it readily enters the air and reaches the moist surface of the eye.
When the compound reaches the eye, sensory nerves register irritation. The lacrimal glands respond by producing more tears, which dilute and wash away the irritant. Crying over an onion is therefore a protective cleaning response.
To reduce the tears, reduce the amount of irritant that reaches your eyes. A sharp knife crushes fewer cells, and airflow from an extractor fan or open window can carry the volatile compound away from your face. Chilling the onion may slow the reactions and evaporation somewhat, although freezing can change its texture.
Onion tears are the meeting point between the plant's chemical defenses and the eye's rinsing system. A microscopic reaction that begins as cells split on the cutting board becomes a visible tear only seconds later.
