They’re essentially thin pieces of material that are stuck to the front surfaces of your teeth. You usually get kits that contain strips for both the top and bottom row of teeth. The whole idea is that the strip focuses on the front surfaces, which are most visible to people when you smile. What these strips do is when you attach them to the teeth, they hold those chemicals against the tooth surface and try to remove the external stains that have discolored your teeth. Your diet and lifestyle will have affected the color of your natural teeth. If you’ve got stains, these whitening strips can potentially remove those external stains. With very few exceptions to any whitening strips contain bleaches or peroxides that can change the natural tooth color. So if you have yellow teeth as a result of aging, maybe you’ve got trauma to your teeth.