Summarize:

They're notorious for making people tear up when they’re sliced — which means they’re a quick and easy way for you to start producing tears for the weepy look of the bedeviled pink eye.  Onions produce a sulfur compound called syn-propanethial-S-oxide. When an onion is cut open, the compound is released into the air. When the chemical reaches your eyes, the lachrymal glands above the eyelids (responsible for the regulation of tears) produce tears to help wash away the irritating chemical.  Sweet onions produce less syn-propanethial-S-oxide than other onion varieties because the sugar and high water content can abate the irritating enzymes. So if you’re really looking to get the tears going, chop the pungent red or white onions instead of the sweet yellow Vidalias. If you aren't trying to get too creative with your tear production techniques, then get some eye drops from the drugstore or supermarket. Usually, directions on eye drop containers suggest only one or two spurts into the eye. To create the illusion of more tears, put a few extra drops than you normally would into your eyes and let them run down your face. Don’t wipe the drops off your cheeks so that everyone will assume your eyes are in a constant state of weeping. Ever wonder how actors manage to cry on the spot? They could just be really good at their jobs... or they could also be using a menthol stick. Menthol sticks are waxy substances that come in lipstick tubes. To use it, you rub the waxy menthol beneath your eyes and then wait for your eyes to water. These are commonly used in theater productions to create realistic crying scenes. If you’re gonna act like you have pink eye, why not really give in to the performance and do as the professionals do?
Chop some onions. Use eye drops. Use a menthol stick.