Why do cutting onions make you cry?

1. Is the crying caused by eye irritation or emotions?

2. Why does it happen?

3. How do you prevent this?

3.1. Under what circumstance will it not occur?

3.1.1. Will Onions that are taken out from a fridge be less irritating to the eyes?

3.1.2. Would there be any changes if any form of impurities is added?

3.1.3. Will different styles of cutting affect the irritation?

4. Does only cutting the onion make you cry?