If you get your teeth whitened at the dentist or buy some over-the-counter whitening products, the active ingredient is usually hydrogen peroxide. The hydrogen peroxide reacts with the colored molecules that are staining your teeth to remove them or at least make them colorless. This can be a slow reaction, so at the dentist’s office, sometimes they use a bright light to speed up the breakdown of the H₂O₂.

If you’re wondering about whitening toothpastes, those contain an abrasive compound that simply rubs the stain molecules off of your teeth—no fancy chemistry at all.