What is the reason for tears coming in the eyes when chopping onions?

There are some compounds of sulphur in onion which can be compared with adulterated acid. These compounds have property of vapourisation. So, when an onion is being chopped vapours of these compounds spread in the air. They cause much irritation in the eyes and nose than any other part of the body. Since these compounds are soluble in water, they dissolve in tears and eventually get flushed out. If the onions are chopped while immersed in water the acidic sulphur compounds immediately dissolve in water and do not spread in air.