Why do our eyes sometimes make tears when we cut onions?

When an onion is cut, damaged cells release enzymes that convert sulfur compounds into a volatile irritant called syn-propanethial-S-oxide. This gas drifts into the eyes and triggers sensory nerves in the cornea. The nerves tell the lacrimal glands to flood the surface with tears, which helps dilute and flush out the irritant. Chilling onions or using a sharp knife can reduce the amount of irritant released.