Why do onions make us cry?
For grown-ups: the science
Cutting an onion makes a gas that turns into a mild acid in your eye water, so your eyes make tears to wash it away.
Cutting ruptures onion cells, allowing the enzyme alliinase to react with sulfur-containing compounds and produce syn-propanethial S-oxide. This volatile compound reaches the eye, dissolves in the tear film, and forms a mild sulfuric acid that irritates the ocular surface and triggers reflex tearing from the lacrimal glands.
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Transcript
A cut onion sits on the cutting board. Why can your eyes start making tears?
When an onion is cut, many tiny living blocks called cells break open. A cell helper mixes with sulfur chemicals and makes a gas.
The gas drifts up and dissolves in the thin water layer on your eyes. There it makes a mild acid that bothers the eye's surface.
Your eyes send a quick signal to tear glands, which make extra tears. The tears wash the mild acid away, so your eyes can feel comfortable again.
So, cutting an onion starts a tiny chemical change. Your tears help rinse the new acid away.