Over time, your teeth will inevitably tend to age and become stained by foodstuffs, particularly by smoking and drinking coffee, tea and red wine. Teeth are porous and stains can become embedded within them, no matter how much you brush you cannot lift these stains out. The basic principle of how teeth whitening works is the specially formulated gel which is applied to the teeth oxidizes against the tooth into tiny molecules which are smaller than the pores on the teeth. These molecules enter the pores and eradicate the staining. The gels can come in different concentrations for different types of whitening.