Teeth whitening uses bleaching gel to whiten the teeth. In-office teeth whitening has the dentist place bleaching gel on the teeth. A light or laser activates the bleaching process. Oxygen emerges into the enamel and lightens the color of the teeth. At home whitening can be performed after the dentist makes a mouth guard for the patient's teeth. The whitening gel is applied to the mouthpiece and inserted into the patient's mouth over the next 2-4 weeks for 30 minutes per day. Over the counter whitening gels are applied to the teeth, twice a day, with a small brush. Whitening strips bought at a store are thin strips of plastic coated with a gel and used twice daily.