Alllll riiight!!! I give up! No one has said anything, so please tell me what a retainer is if it isn't money or a family employee, and how you get it tea-stained? And what for? Does it hurt?
Oh... just as I sent that off, I saw the teeth tag.
I assume from your wording that it is a tooth brace to correct the set of your teeth. I also assume that you sleep with it in your mouth. I assume besides, that it picked up tea stains during the previous day.
So what you have at night is a metal object with brown tea-stain, and in the morning is a disappointingly pale shadow of the handsome creation of the night before?
The stainig is caused by brown chemicals in the tea that have parts sticking out like the suckers on Virginia creeper. Those parts are largely what chemists call acidic groups and phenolic groups. The brown chemicals are what chemists call polyphenols and what human beings call tannins (not always correctly, but it will do for now.)
Your saliva contains water and also a range of other molecules, mainly proteins. They don't do explosive things, but they are very, very persistent and they do erode practically anything you put into your mouth, enough to drive a dentist to desperation. They pile onto the polyphenols in several ways and every now and then they can pry a bit off the metal and slip in between the metal and the polyphenol. Whenever that happens they clog the acid or phenolic groups so that they no longer stick to other things, and sooner or later the polyphenol peels off the metal and you swallow it. It is a little like soaking painted wood in soapy water. Sooner or later, if the water finds a crack, it will make the paint start to peel.
By morning, when you are ready to brush your teeth, your retainer has ceased to retain your tea-stains. Just let's hope it still retains your teeth.
