Onions contains a compound known as “Lachrymator” which when released makes the nerves near the eyes irate.When the onion is sliced the lachrymator is released in the air & the enzyme helps in altering the amino acids into lachrymator compounds & thus it hurts the eyes & it makes us cry.Well,to prevent this always use a sharp knife to slice the onion,keep the onion in refrigerator for few hours before slicing it,cut the onion into 2 pieces & soak it in water for at-least 20 minutes before slicing it,switch on the fan or ventilator while slicing so that the allyl sulphide is blown away in air.
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