Why Cutting an Onion Creates an Eye Irritant

A knife cuts an onion, yet the reaction can happen at your eyes. Follow the hidden connection: damaged plant cells, two enzyme steps, a volatile tear factor, and the mechanics of a blade opening wet tissue. We distinguish individual molecules in air from liquid droplets, explain what the 2025 high-speed cutting study measured, and separate chemical evidence from what an ordinary cooking video can show. The enlarged cells, reaction routes, layer cutaway and particles are original explanatory models. Drawn geometry, particle sizes and animation timing are illustrative; they are not microscopic recordings, tracked experimental particles or fitted numerical simulations. CHAPTERS 0:00 The cut starts a reaction 0:49 An onion is organized tissue 1:45 Two enzymes do different jobs 2:40 Test the chemical switch 3:39 A liquid surface can supply vapor 4:34 The blade also deforms tissue 5:22 One cut can make two kinds of spray 6:18 What a camera proves 7:14 Arrival triggers a response 8:09 Two stories meet at the cutting board PRIMARY SCIENTIFIC SOURCES Imai et al. (2002), An onion enzyme that makes the eyes water: https://www.nature.com/articles/419685a Silvaroli et al. (2017), Enzyme That Makes You Cry—Crystal Structure of Lachrymatory Factor Synthase from Allium cepa: https://pmc.ncbi.nlm.nih.gov/articles... Eady et al. (2008), Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile: https://pmc.ncbi.nlm.nih.gov/articles... Wu et al. (2025), high-speed onion cutting mechanics: https://doi.org/10.1073/pnas.2512779122 TFOS DEWS II Pathophysiology report (ocular surface / reflex tearing context): https://www.tfosdewsreport.org/report... MOVING FOOTAGE AND CREDITS Ron Lach / Pexels, person cutting an onion: https://www.pexels.com/video/a-person... Los Muertos Crew / Pexels, person cutting an onion: https://www.pexels.com/video/person-c... Both used as edited footage under the Pexels License: https://www.pexels.com/license/ Original Blender layer model, reaction diagrams, motion graphics and explanatory editing: Matter Made Clear. No figures or supplementary movies from the 2025 paper are reproduced. An educational explanation of chemistry and mechanics, not a test of the depicted cook's eye response.