Sink stinks

So if you let the water out of our sink it totally smells bad. Anyone have any idea why this might be? Is it as simple as putting Draino down the sink or is there something else that I should try first? I care but its the girls I'm worried about because I'm leaving soon so I hope that maybe I can fix it before they go away. I know a guy who works in the sink business I wonder if he maybe able to help me.