The source provides a comprehensive explanation of why slicing onions causes tears, detailing the underlying chemical, biological, and evolutionary processes. It explains that when an onion's cells are ruptured, an enzyme called alliinase reacts with sulfur compounds to produce propanethial S-oxide, a volatile gas known as the lachrymatory factor (LF). When this gas reaches the eyes, it forms sulfuric acid, triggering a protective tearing response from the human body designed to flush out the irritant. Furthermore, the episode explores the onion's evolutionary history, noting that these compounds serve as a defense mechanism against predators and pathogens, and discusses various strategies and scientific efforts—such as chilling, ventilation, and genetic modification—to minimize or prevent the tear-inducing effect.

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