You asked:
Why do you cry when you cut onions? — Peterson, Visakhapatnam, India

Marshall Brain Answered:
The short answer is “sulfur”. Onions capture sulfur from the soil and use it to create amino acids, and this sulfur is the ultimate source of the tears.

But all living things contain sulfur in their amino acids – the sulfur dioxide given off by dead animals is a big part of their bad smell. There is even a sulfur cycle in biology where the sulfur released by dying animals becomes the sulfur of new animals and microorganisms.

The key thing about onions is an enzyme called lachrymatory-factor synthase. This enzyme causes a chain reaction that turns the sulfur into syn-propanethial-S-oxide – described here a “combination of sulfuric acid, sulfur dioxide and hydrogen sulfide”. These are the chemicals that cause the tears.

Now that scientists know about lachrymatory-factor synthase, they are trying to create a new species of onion that does not contain it. If they are successful, we will have tearless onions.

See also: Why do onions make you cry?

A handy tip to avoid onion tears: