Activated Charcoal
To make it simple, activated charcoal pulls toxins from the mouth and removes stains. It is also helpful in changing the pH and health of your mouth and assists in preventing cavities and adsorbing the bad bacteria present that causes tooth decay and gingivitis. The reason why it helps whiten teeth over a period of time is that, "[Activated charcoal] is shown to be attracted to a group of compounds known as tannins. Tannins can be found in common stain-inducing items like wine or coffee, for instance."