Question

When our mother cut the onion in the room ,then our tears are fallen down from our eyes.Why?

Answer 1

when you cut an onion, it releases a gas called, Propanethiol S-oxide. When mixed with certain enzymes in the onion, it creates a sulphur  gas. These gases then get to your eyes and create a mild acid which irritates the eyes . 

Answer 2

the cut onion releases a gaseous chemical which is an eye irritant. to excrete that irritant the eye lubricant tear cleans our eye and when the tears escape our eyes it takes the irritant with it. tears continue as long as the cut onion is present near us.