Onions contain a poison that is released into the air when they are cut open and causes eye watering in humans. In smaller amount, we enjoy the taste of this toxin. It is more toxic to some other animals, despite being fairly benign to humans.
There probably are multiple versions of this toxin, or multiple toxins. Leeks and garlic are basically different kinds of onions, but they taste different enough (due to said chemicals) that we call them by different names. I don't know whether the types of poisons they contain are different, or if they contain the same things but in different amounts.
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