Below the enamel is the dentin layer, which is the one that gives the tooth its color. It is here that your teeth get stained. The enamel layer is porous, so staining particles from coffee, tobacco, wine, tea, etc. can seep through these pores to reach the dentin, and that’s where they accumulate. The more of these particles get through, the more discolored (stained) your teeth become.
In order to whiten your teeth, you need to get a bleaching substance to the dentin layer. The best way to bleach anything is with oxygen radicals, but you can’t pump pure oxygen (or especially oxygen free radicals) to your teeth, much less through the enamel. The best-known teeth whitening chemical in the world at the time this was written is hydrogen peroxide. Hydrogen peroxide (H2O2) is NOT the same as oxygen (O2), but when hydrogen peroxide reacts, it breaks down into water (H2O) and oxygen radical (of which there are a few varieties). Oxygen radicals only live a fraction of a second, but in that time they desperately look for something to oxidize (or react with), and the stains on your teeth are just what the doctor ordered.
When hydrogen peroxide or carbamide peroxide gel is applied to the teeth, the hydrogen peroxide breaks down on the tooth’s surface and travels through the pores in the enamel of the tooth until it reaches the dentin layer. There, the oxygen radicals react with the stained particles on the dentin and whiten them. They do NOT remove the staining molecules, but simply bleach them, removing their color. If you do this long and often enough with the appropriate strength of hydrogen peroxide gel (or some derivative form of it, such as carbamide peroxide), your teeth will appear significantly whiter.