The surface of the teeth contains millions of microscopic pores, and

over the years, organic compounds from food, drink and tobacco penetrate these pores causing discoloration. Tooth whitening occurs when hydrogen peroxide breaks down into oxygen molecules, which penetrate through the enamel and go into the dentin, which is  directly under the enamel. These molecules go into the tiny pores of the enamel and dentin and bleach the colored substances.