Your eyes start to water when you're cutting an onion because of propanethial s-oxide, which your eyes react to. Cutting onions under water can prevent this from happening.
Eating your food slowly helps you eat less (and lose weight). This is because it takes 20 minutes for your gut to suppress a hormone called ghrelin, and release anti-hunger hormones. So eating slowly gives your brain the time it needs to receive these signals.