Everyone’s mother seems to have their own unique proposal about how to avoid crying over diced onions, but there’s only one that makes any chemical sense (excluding wearing a gas mask): put the onion in the fridge before you cut it. Almost every chemical reaction is slower at lower temperature, and since the lachrymator in onions is produced when you cut it (and not naturally present in the onion), you can give yourself more tear-free time to dice if the onion is cold.