So a woman goes to the doctor with a busted lip and a black eye...

...and the doctor says, "Oh my, what happened?"
To which the woman replies, "Well my husband always goes to the bar after work, he then proceeds to come home and beat me. I've tried everything to get him to stop but nothing works." The doc listens to her story and after a few minutes, he responds with "Hmmm, welp, I think I may be able to help you. From now on, the second your husband walks through that door I want you to drink some water but keep some of that water in your mouth and swoosh through your teeth until he goes to bed. Do that and I promise he'll never lay a hand on you, ever." The woman, although slightly confused, thanks the doc and leaves.
Later on that night the husband arrives reeking of alcohol. Taking the doctors advice, she guzzles some water and starts swooshing it between her teeth until he goes to bed. To her amazement, it works so she repeats the process for a few weeks. Completely baffled, she returns to the doctors office to quiz him on this "cure". He then says to the woman, "that's called keeping your mouth shut."