Well, if your answer to my question is a 'No', then that means you have never cut an onion in your life. Yes, sulfuric acid is the villain that makes you cry while cutting onions.
Let's talk some chemistry this time. When you cut onions, you are breaking the cells in it which releases a sulfur containing chemical called amino acid sulfoxide and some enzymes. These enzymes convert the sulfoxides into sulfenic acid, which is a highly unstable compound. It is suddenly converted into a gas called propanethiol S-oxide. It is a low density gas and so wafts up in the air soon. As you know, our eyes always produce tears necessary to prevent our eyes from going dry. When the propanethiol S-oxide reacts with the water in our eyes, caustic sulfuric acid is formed. Can our brain wait anymore? Before sulfuric acid burns our eyes, brain give order to wash off this villain and we start crying.