If your eyes tear when you cut onions here are a few things to try:
Soak the onion in cold water before cutting it. The cold will chill the gases and lessen their impact.
Contact lenses may block the gases and make them sting less.
If you don’t wear contact lenses or glasses there is one more trick that may work. It sounds crazy but many people swear by it. Try sticking a piece of bread in your mouth while you slice. The bread will absorb some of the gases before they rise up to your nose and eyes. Strange but true!