Cutting an onion can make your eyes hurt and fill with tears. This is not only because onions smell strong. A small chemical reaction happens inside the onion when it is cut.
When a knife breaks onion cells, substances inside the onion mix together. They make a chemical that moves into the air. When this chemical reaches your eyes, your eyes try to protect themselves.
Tears help wash the chemical away. This does not mean the onion is making us sad. Our eyes are simply protecting themselves from something irritating.
A sharp knife can break fewer cells and may make the reaction smaller.