Although onions taste really delicious on hamburgers or in a stew, the person cutting and cooking them usually sheds a great many tears first.
This happens because onions contain a strong smelling oil.
When the onion is peeled and cut, this oil escapes into the air in the form of vapor, or tiny particles of moisture.
When this vapor reaches your nose, the molecules get into the sensitive membranes lining your nostrils and irritate them.
This, in turn, irritates the tear ducts in your eyes and they produce tears.