tears

An intact onion does nothing to your eyes. But cutting one releases enzymes that act on sulfur compounds, triggering a chemical cascade that produces syn-propanethial-S-oxide — a volatile gas that irritates your cornea and forces your eyes to water. The science behind this everyday kitchen reaction reveals how plants weaponize chemistry as a defense mechanism.

Tears are among the most visible and emotionally laden expressions in human behavior. They fall from the eyes during emotional moments — joy, sadness, grief, anger, or profound relief — yet tears also serve practical functions: lubricating the eyes during blinking, protecting the cornea from irritants, and fighting infection. Humans produce three types of tears, each with a distinct chemical composition and function. Basal tears are produced continuously throughout the day to maintain corneal health. Reflex tears flow in response to irritants like onions, cold wind, or dust. Emotional tears, uniquely abundant in humans compared to other primates, contain elevated levels of stress hormones including cortisol and prolactin, suggesting that crying is a physiological response to emotional stress. Research indicates that emotional crying may indeed relieve stress, though the evidence is more nuanced than pop-psychology claims. The tear-related hypothesis — that the physical act of crying somehow removes stress from the body — has become increasingly credible as neuroscience reveals the neurochemical basis of emotional tears and their relationship to the sympathetic nervous system, emotion regulation, and social signaling.

Intense laughter can make people cry real tears. Here's the physiological mechanism, the brain science, and why joy and grief share a face.

Three types of tears, the chemistry of emotional crying, and why humans are the only species that weeps from feelings. The science of tears explained.