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Why do eyes tear when we work with onions?

When an onion is cut, its pierced cells release a sulfur compound, thio-propanal-s-oxide, through a series of rapid chemical reactions. This substance is irritating to the eyes.

Sources: McGrayne, Sharon Bertsch. 365 Surprising Scientific Facts, Breakthroughs, and Discoveries, p. 132; Pittsburgh Post Gazette, January 22,1995, p. JI.

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1. Characteristics And Functions
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   2. What is the normal pH of blood, urine, and saliva?
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   4. How does the body introduce oxygen to the blood and where does this happen?
   5. How does the immune system work?
   6. How much blood is in the average human body?
   7. What are the seven endocrine glands?
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   9. How many miles of blood vessels are contained in the body?
   10. How many chromosomes are there in a human body cell?
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   14. What regulates body temperature in humans?
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   16. What percent of human body weight is water?
   17. What are considered healthy weights for men and women?
   18. Who is the heaviest person that ever lived?
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