Ever wondered why cutting onions makes you cry?

As you slice an onion, the onion cells release a gas into the air.  When this gas contacts the eye, it turns into weak sulphuric acid in the moist tear layer that coats the eye.  Nerve endings in the cornea react to the acid, causing the sting, and trigger the lacrimal (tear) glands to produce lots of tears in an attempt to further dilute and flush away the acid. 

If onions are in water, or near running water when cut, there will be much less gas in the air, and you are less likely to start crying.