Why we cry when cutting onions?

If we cut onions, automatically tears will come out of our eyes. What's the reason behind these?

 When we cut onions, chemicals such as sulfoxides and enzymes are produced. These may combine and react to form a gas. These gas reacts with lubricating liquid in our eyes. As a result of this, the sulphuric acid is produced which is responsible for the tears. If we store onion in the refrigerator, the temperature of the onion falls down and the reaction between the sulfoxides and enzymes take place slowly and the resultant gas is not so stronger. It is also said that the concentration of  chemicals is more in the bulbs. Hence, in order to avoid crying, we should cut the bulbs first. Also, cutting an onion under the running water washes away the chemicals. Hence we can also use this method to avoid crying. We can also wear tight goggles to avoid the gas to reach our eyes.