easiest way is to wash the onion under cold water after cutting it in half. Gets rid of the chemical that causes your eyes to tear

Also, working near an open flame creates a convection current to the flame where the gas that causes tears are incinerated. Onions cause tears because when they are cut, when their cell walls are breached, a gas is formed that rises. When it hits our eyes, it combines with our tears to form sulferic acid. No wonder it hurts!

An other way to help stop the tears from rolling is to stick your head in the freezer every time your eyes start to burn from the onions.